BCP 47 - Tags for Identifying Languages

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    [Note that this file is a concatenation of more than one RFC.]

    Network Working Group                                   A. Phillips, Ed.
    Request for Comments: 5646                                        Lab126
    BCP: 47                                                    M. Davis, Ed.
    Obsoletes: 4646                                                   Google
    Category: Best Current Practice                           September 2009

                         Tags for Identifying Languages

    Abstract

       This document describes the structure, content, construction, and
       semantics of language tags for use in cases where it is desirable to
       indicate the language used in an information object.  It also
       describes how to register values for use in language tags and the
       creation of user-defined extensions for private interchange.

    Status of This Memo

       This document specifies an Internet Best Current Practices for the
       Internet Community, and requests discussion and suggestions for
       improvements.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (c) 2009 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

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       Please review these documents carefully, as they describe your rights
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       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

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    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.  The Language Tag . . . . . . . . . . . . . . . . . . . . . . .  4
         2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
           2.1.1.  Formatting of Language Tags  . . . . . . . . . . . . .  6
         2.2.  Language Subtag Sources and Interpretation . . . . . . . .  8
           2.2.1.  Primary Language Subtag . . . . . . . . . . . . . . . . 9
           2.2.2.  Extended Language Subtags  . . . . . . . . . . . . . . 11
           2.2.3.  Script Subtag  . . . . . . . . . . . . . . . . . . . . 12
           2.2.4.  Region Subtag  . . . . . . . . . . . . . . . . . . . . 13
           2.2.5.  Variant Subtags  . . . . . . . . . . . . . . . . . . . 15
           2.2.6.  Extension Subtags  . . . . . . . . . . . . . . . . . . 16
           2.2.7.  Private Use Subtags  . . . . . . . . . . . . . . . . . 18
           2.2.8.  Grandfathered and Redundant Registrations  . . . . . . 18
           2.2.9.  Classes of Conformance . . . . . . . . . . . . . . . . 19
       3.  Registry Format and Maintenance  . . . . . . . . . . . . . . . 21
         3.1.  Format of the IANA Language Subtag Registry  . . . . . . . 21
           3.1.1.  File Format  . . . . . . . . . . . . . . . . . . . . . 21
           3.1.2.  Record and Field Definitions . . . . . . . . . . . . . 23
           3.1.3.  Type Field . . . . . . . . . . . . . . . . . . . . . . 26
           3.1.4.  Subtag and Tag Fields  . . . . . . . . . . . . . . . . 26
           3.1.5.  Description Field  . . . . . . . . . . . . . . . . . . 26
           3.1.6.  Deprecated Field . . . . . . . . . . . . . . . . . . . 28
           3.1.7.  Preferred-Value Field  . . . . . . . . . . . . . . . . 28
           3.1.8.  Prefix Field . . . . . . . . . . . . . . . . . . . . . 31
           3.1.9.  Suppress-Script Field  . . . . . . . . . . . . . . . . 32
           3.1.10. Macrolanguage Field  . . . . . . . . . . . . . . . . . 32
           3.1.11. Scope Field  . . . . . . . . . . . . . . . . . . . . . 33
           3.1.12. Comments Field . . . . . . . . . . . . . . . . . . . . 34
         3.2.  Language Subtag Reviewer . . . . . . . . . . . . . . . . . 35
         3.3.  Maintenance of the Registry  . . . . . . . . . . . . . . . 35
         3.4.  Stability of IANA Registry Entries . . . . . . . . . . . . 36
         3.5.  Registration Procedure for Subtags . . . . . . . . . . . . 41
         3.6.  Possibilities for Registration . . . . . . . . . . . . . . 46
         3.7.  Extensions and the Extensions Registry . . . . . . . . . . 49
         3.8.  Update of the Language Subtag Registry . . . . . . . . . . 52
         3.9.  Applicability of the Subtag Registry . . . . . . . . . . . 52
       4.  Formation and Processing of Language Tags  . . . . . . . . . . 53
         4.1.  Choice of Language Tag . . . . . . . . . . . . . . . . . . 53
           4.1.1.  Tagging Encompassed Languages  . . . . . . . . . . . . 58
           4.1.2.  Using Extended Language Subtags  . . . . . . . . . . . 59
         4.2.  Meaning of the Language Tag  . . . . . . . . . . . . . . . 61
         4.3.  Lists of Languages . . . . . . . . . . . . . . . . . . . . 63
         4.4.  Length Considerations  . . . . . . . . . . . . . . . . . . 63
           4.4.1.  Working with Limited Buffer Sizes  . . . . . . . . . . 64
           4.4.2.  Truncation of Language Tags  . . . . . . . . . . . . . 65
         4.5.  Canonicalization of Language Tags  . . . . . . . . . . . . 66

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         4.6.  Considerations for Private Use Subtags . . . . . . . . . . 68
       5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 69
         5.1.  Language Subtag Registry . . . . . . . . . . . . . . . . . 69
         5.2.  Extensions Registry  . . . . . . . . . . . . . . . . . . . 71
       6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 71
       7.  Character Set Considerations . . . . . . . . . . . . . . . . . 72
       8.  Changes from RFC 4646  . . . . . . . . . . . . . . . . . . . . 73
       9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 76
         9.1.  Normative References . . . . . . . . . . . . . . . . . . . 76
         9.2.  Informative References . . . . . . . . . . . . . . . . . . 78
       Appendix A.  Examples of Language Tags (Informative) . . . . . . . 80
       Appendix B.  Examples of Registration Forms  . . . . . . . . . . . 82
       Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 83

    1.  Introduction

       Human beings on our planet have, past and present, used a number of
       languages.  There are many reasons why one would want to identify the
       language used when presenting or requesting information.

       The language of an information item or a user's language preferences
       often need to be identified so that appropriate processing can be
       applied.  For example, the user's language preferences in a Web
       browser can be used to select Web pages appropriately.  Language
       information can also be used to select among tools (such as
       dictionaries) to assist in the processing or understanding of content
       in different languages.  Knowledge about the particular language used
       by some piece of information content might be useful or even required
       by some types of processing, for example, spell-checking, computer-
       synthesized speech, Braille transcription, or high-quality print
       renderings.

       One means of indicating the language used is by labeling the
       information content with an identifier or "tag".  These tags can also
       be used to specify the user's preferences when selecting information
       content or to label additional attributes of content and associated
       resources.

       Sometimes language tags are used to indicate additional language
       attributes of content.  For example, indicating specific information
       about the dialect, writing system, or orthography used in a document
       or resource may enable the user to obtain information in a form that
       they can understand, or it can be important in processing or
       rendering the given content into an appropriate form or style.

       This document specifies a particular identifier mechanism (the
       language tag) and a registration function for values to be used to

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       form tags.  It also defines a mechanism for private use values and
       future extensions.

       This document replaces [RFC4646] (which obsoleted [RFC3066] which, in
       turn, replaced [RFC1766]).  This document, in combination with
       [RFC4647], comprises BCP 47.  For a list of changes in this document,
       see Section 8.

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

    2.  The Language Tag

       Language tags are used to help identify languages, whether spoken,
       written, signed, or otherwise signaled, for the purpose of
       communication.  This includes constructed and artificial languages
       but excludes languages not intended primarily for human
       communication, such as programming languages.

    2.1.  Syntax

       A language tag is composed from a sequence of one or more "subtags",
       each of which refines or narrows the range of language identified by
       the overall tag.  Subtags, in turn, are a sequence of alphanumeric
       characters (letters and digits), distinguished and separated from
       other subtags in a tag by a hyphen ("-", [Unicode] U+002D).

       There are different types of subtag, each of which is distinguished
       by length, position in the tag, and content: each subtag's type can
       be recognized solely by these features.  This makes it possible to
       extract and assign some semantic information to the subtags, even if
       the specific subtag values are not recognized.  Thus, a language tag
       processor need not have a list of valid tags or subtags (that is, a
       copy of some version of the IANA Language Subtag Registry) in order
       to perform common searching and matching operations.  The only
       exceptions to this ability to infer meaning from subtag structure are
       the grandfathered tags listed in the productions 'regular' and
       'irregular' below.  These tags were registered under [RFC3066] and
       are a fixed list that can never change.

       The syntax of the language tag in ABNF [RFC5234] is:

     Language-Tag  = langtag             ; normal language tags
                   / privateuse          ; private use tag
                   / grandfathered       ; grandfathered tags

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     langtag       = language
                     ["-" script]
                     ["-" region]
                     *("-" variant)
                     *("-" extension)
                     ["-" privateuse]

     language      = 2*3ALPHA            ; shortest ISO 639 code
                     ["-" extlang]       ; sometimes followed by
                                         ; extended language subtags
                   / 4ALPHA              ; or reserved for future use
                   / 5*8ALPHA            ; or registered language subtag

     extlang       = 3ALPHA              ; selected ISO 639 codes
                     *2("-" 3ALPHA)      ; permanently reserved

     script        = 4ALPHA              ; ISO 15924 code

     region        = 2ALPHA              ; ISO 3166-1 code
                   / 3DIGIT              ; UN M.49 code

     variant       = 5*8alphanum         ; registered variants
                   / (DIGIT 3alphanum)

     extension     = singleton 1*("-" (2*8alphanum))

                                         ; Single alphanumerics
                                         ; "x" reserved for private use
     singleton     = DIGIT               ; 0 - 9
                   / %x41-57             ; A - W
                   / %x59-5A             ; Y - Z
                   / %x61-77             ; a - w
                   / %x79-7A             ; y - z

     privateuse    = "x" 1*("-" (1*8alphanum))

     grandfathered = irregular           ; non-redundant tags registered
                   / regular             ; during the RFC 3066 era

     irregular     = "en-GB-oed"         ; irregular tags do not match
                   / "i-ami"             ; the 'langtag' production and
                   / "i-bnn"             ; would not otherwise be
                   / "i-default"         ; considered 'well-formed'
                   / "i-enochian"        ; These tags are all valid,
                   / "i-hak"             ; but most are deprecated
                   / "i-klingon"         ; in favor of more modern
                   / "i-lux"             ; subtags or subtag
                   / "i-mingo"           ; combination

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                   / "i-navajo"
                   / "i-pwn"
                   / "i-tao"
                   / "i-tay"
                   / "i-tsu"
                   / "sgn-BE-FR"
                   / "sgn-BE-NL"
                   / "sgn-CH-DE"

     regular       = "art-lojban"        ; these tags match the 'langtag'
                   / "cel-gaulish"       ; production, but their subtags
                   / "no-bok"            ; are not extended language
                   / "no-nyn"            ; or variant subtags: their meaning
                   / "zh-guoyu"          ; is defined by their registration
                   / "zh-hakka"          ; and all of these are deprecated
                   / "zh-min"            ; in favor of a more modern
                   / "zh-min-nan"        ; subtag or sequence of subtags
                   / "zh-xiang"

     alphanum      = (ALPHA / DIGIT)     ; letters and numbers

                            Figure 1: Language Tag ABNF

       For examples of language tags, see Appendix A.

       All subtags have a maximum length of eight characters.  Whitespace is
       not permitted in a language tag.  There is a subtlety in the ABNF
       production 'variant': a variant starting with a digit has a minimum
       length of four characters, while those starting with a letter have a
       minimum length of five characters.

       Although [RFC5234] refers to octets, the language tags described in
       this document are sequences of characters from the US-ASCII [ISO646]
       repertoire.  Language tags MAY be used in documents and applications
       that use other encodings, so long as these encompass the relevant
       part of the US-ASCII repertoire.  An example of this would be an XML
       document that uses the UTF-16LE [RFC2781] encoding of [Unicode].

    2.1.1.  Formatting of Language Tags

       At all times, language tags and their subtags, including private use
       and extensions, are to be treated as case insensitive: there exist
       conventions for the capitalization of some of the subtags, but these
       MUST NOT be taken to carry meaning.

       Thus, the tag "mn-Cyrl-MN" is not distinct from "MN-cYRL-mn" or "mN-
       cYrL-Mn" (or any other combination), and each of these variations

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       conveys the same meaning: Mongolian written in the Cyrillic script as
       used in Mongolia.

       The ABNF syntax also does not distinguish between upper- and
       lowercase: the uppercase US-ASCII letters in the range 'A' through
       'Z' are always considered equivalent and mapped directly to their US-
       ASCII lowercase equivalents in the range 'a' through 'z'.  So the tag
       "I-AMI" is considered equivalent to that value "i-ami" in the
       'irregular' production.

       Although case distinctions do not carry meaning in language tags,
       consistent formatting and presentation of language tags will aid
       users.  The format of subtags in the registry is RECOMMENDED as the
       form to use in language tags.  This format generally corresponds to
       the common conventions for the various ISO standards from which the
       subtags are derived.

       These conventions include:

       o  [ISO639-1] recommends that language codes be written in lowercase
          ('mn' Mongolian).

       o  [ISO15924] recommends that script codes use lowercase with the
          initial letter capitalized ('Cyrl' Cyrillic).

       o  [ISO3166-1] recommends that country codes be capitalized ('MN'
          Mongolia).

       An implementation can reproduce this format without accessing the
       registry as follows.  All subtags, including extension and private
       use subtags, use lowercase letters with two exceptions: two-letter
       and four-letter subtags that neither appear at the start of the tag
       nor occur after singletons.  Such two-letter subtags are all
       uppercase (as in the tags "en-CA-x-ca" or "sgn-BE-FR") and four-
       letter subtags are titlecase (as in the tag "az-Latn-x-latn").

       Note: Case folding of ASCII letters in certain locales, unless
       carefully handled, sometimes produces non-ASCII character values.
       The Unicode Character Database file "SpecialCasing.txt"
       [SpecialCasing] defines the specific cases that are known to cause
       problems with this.  In particular, the letter 'i' (U+0069) in
       Turkish and Azerbaijani is uppercased to U+0130 (LATIN CAPITAL LETTER
       I WITH DOT ABOVE).  Implementers SHOULD specify a locale-neutral
       casing operation to ensure that case folding of subtags does not
       produce this value, which is illegal in language tags.  For example,
       if one were to uppercase the region subtag 'in' using Turkish locale
       rules, the sequence U+0130 U+004E would result, instead of the
       expected 'IN'.

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    2.2.  Language Subtag Sources and Interpretation

       The namespace of language tags and their subtags is administered by
       the Internet Assigned Numbers Authority (IANA) according to the rules
       in Section 5 of this document.  The Language Subtag Registry
       maintained by IANA is the source for valid subtags: other standards
       referenced in this section provide the source material for that
       registry.

       Terminology used in this document:

       o  "Tag" refers to a complete language tag, such as "sr-Latn-RS" or
          "az-Arab-IR".  Examples of tags in this document are enclosed in
          double-quotes ("en-US").

       o  "Subtag" refers to a specific section of a tag, delimited by a
          hyphen, such as the subtags 'zh', 'Hant', and 'CN' in the tag "zh-
          Hant-CN".  Examples of subtags in this document are enclosed in
          single quotes ('Hant').

       o  "Code" refers to values defined in external standards (and that
          are used as subtags in this document).  For example, 'Hant' is an
          [ISO15924] script code that was used to define the 'Hant' script
          subtag for use in a language tag.  Examples of codes in this
          document are enclosed in single quotes ('en', 'Hant').

       Language tags are designed so that each subtag type has unique length
       and content restrictions.  These make identification of the subtag's
       type possible, even if the content of the subtag itself is
       unrecognized.  This allows tags to be parsed and processed without
       reference to the latest version of the underlying standards or the
       IANA registry and makes the associated exception handling when
       parsing tags simpler.

       Some of the subtags in the IANA registry do not come from an
       underlying standard.  These can only appear in specific positions in
       a tag: they can only occur as primary language subtags or as variant
       subtags.

       Sequences of private use and extension subtags MUST occur at the end
       of the sequence of subtags and MUST NOT be interspersed with subtags
       defined elsewhere in this document.  These sequences are introduced
       by single-character subtags, which are reserved as follows:

       o  The single-letter subtag 'x' introduces a sequence of private use
          subtags.  The interpretation of any private use subtag is defined

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          solely by private agreement and is not defined by the rules in
          this section or in any standard or registry defined in this
          document.

       o  The single-letter subtag 'i' is used by some grandfathered tags,
          such as "i-default", where it always appears in the first position
          and cannot be confused with an extension.

       o  All other single-letter and single-digit subtags are reserved to
          introduce standardized extension subtag sequences as described in
          Section 3.7.

    2.2.1.  Primary Language Subtag

       The primary language subtag is the first subtag in a language tag and
       cannot be omitted, with two exceptions:

       o  The single-character subtag 'x' as the primary subtag indicates
          that the language tag consists solely of subtags whose meaning is
          defined by private agreement.  For example, in the tag "x-fr-CH",
          the subtags 'fr' and 'CH' do not represent the French language or
          the country of Switzerland (or any other value in the IANA
          registry) unless there is a private agreement in place to do so.
          See Section 4.6.

       o  The single-character subtag 'i' is used by some grandfathered tags
          (see Section 2.2.8) such as "i-klingon" and "i-bnn".  (Other
          grandfathered tags have a primary language subtag in their first
          position.)

       The following rules apply to the primary language subtag:

       1.  Two-character primary language subtags were defined in the IANA
           registry according to the assignments found in the standard "ISO
           639-1:2002, Codes for the representation of names of languages --
           Part 1: Alpha-2 code" [ISO639-1], or using assignments
           subsequently made by the ISO 639-1 registration authority (RA) or
           governing standardization bodies.

       2.  Three-character primary language subtags in the IANA registry
           were defined according to the assignments found in one of these
           additional ISO 639 parts or assignments subsequently made by the
           relevant ISO 639 registration authorities or governing
           standardization bodies:

           A.  "ISO 639-2:1998 - Codes for the representation of names of
               languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2]

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           B.  "ISO 639-3:2007 - Codes for the representation of names of
               languages -- Part 3: Alpha-3 code for comprehensive coverage
               of languages" [ISO639-3]

           C.  "ISO 639-5:2008 - Codes for the representation of names of
               languages -- Part 5: Alpha-3 code for language families and
               groups" [ISO639-5]

       3.  The subtags in the range 'qaa' through 'qtz' are reserved for
           private use in language tags.  These subtags correspond to codes
           reserved by ISO 639-2 for private use.  These codes MAY be used
           for non-registered primary language subtags (instead of using
           private use subtags following 'x-').  Please refer to Section 4.6
           for more information on private use subtags.

       4.  Four-character language subtags are reserved for possible future
           standardization.

       5.  Any language subtags of five to eight characters in length in the
           IANA registry were defined via the registration process in
           Section 3.5 and MAY be used to form the primary language subtag.
           An example of what such a registration might include is the
           grandfathered IANA registration "i-enochian".  The subtag
           'enochian' could be registered in the IANA registry as a primary
           language subtag (assuming that ISO 639 does not register this
           language first), making tags such as "enochian-AQ" and "enochian-
           Latn" valid.

           At the time this document was created, there were no examples of
           this kind of subtag.  Future registrations of this type are
           discouraged: an attempt to register any new proposed primary
           language MUST be made to the ISO 639 registration authority.
           Proposals rejected by the ISO 639 registration authority are
           unlikely to meet the criteria for primary language subtags and
           are thus unlikely to be registered.

       6.  Other values MUST NOT be assigned to the primary subtag except by
           revision or update of this document.

       When languages have both an ISO 639-1 two-character code and a three-
       character code (assigned by ISO 639-2, ISO 639-3, or ISO 639-5), only
       the ISO 639-1 two-character code is defined in the IANA registry.

       When a language has no ISO 639-1 two-character code and the ISO
       639-2/T (Terminology) code and the ISO 639-2/B (Bibliographic) code
       for that language differ, only the Terminology code is defined in the
       IANA registry.  At the time this document was created, all languages
       that had both kinds of three-character codes were also assigned a

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       two-character code; it is expected that future assignments of this
       nature will not occur.

       In order to avoid instability in the canonical form of tags, if a
       two-character code is added to ISO 639-1 for a language for which a
       three-character code was already included in either ISO 639-2 or ISO
       639-3, the two-character code MUST NOT be registered.  See
       Section 3.4.

       For example, if some content were tagged with 'haw' (Hawaiian), which
       currently has no two-character code, the tag would not need to be
       changed if ISO 639-1 were to assign a two-character code to the
       Hawaiian language at a later date.

       To avoid these problems with versioning and subtag choice (as
       experienced during the transition between RFC 1766 and RFC 3066), as
       well as to ensure the canonical nature of subtags defined by this
       document, the ISO 639 Registration Authority Joint Advisory Committee
       (ISO 639/RA-JAC) has included the following statement in
       [iso639.prin]:

          "A language code already in ISO 639-2 at the point of freezing ISO
          639-1 shall not later be added to ISO 639-1.  This is to ensure
          consistency in usage over time, since users are directed in
          Internet applications to employ the alpha-3 code when an alpha-2
          code for that language is not available."

    2.2.2.  Extended Language Subtags

       Extended language subtags are used to identify certain specially
       selected languages that, for various historical and compatibility
       reasons, are closely identified with or tagged using an existing
       primary language subtag.  Extended language subtags are always used
       with their enclosing primary language subtag (indicated with a
       'Prefix' field in the registry) when used to form the language tag.
       All languages that have an extended language subtag in the registry
       also have an identical primary language subtag record in the
       registry.  This primary language subtag is RECOMMENDED for forming
       the language tag.  The following rules apply to the extended language
       subtags:

       1.  Extended language subtags consist solely of three-letter subtags.
           All extended language subtag records defined in the registry were
           defined according to the assignments found in [ISO639-3].
           Language collections and groupings, such as defined in
           [ISO639-5], are specifically excluded from being extended
           language subtags.

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       2.  Extended language subtag records MUST include exactly one
           'Prefix' field indicating an appropriate subtag or sequence of
           subtags for that extended language subtag.

       3.  Extended language subtag records MUST include a 'Preferred-
           Value'.  The 'Preferred-Value' and 'Subtag' fields MUST be
           identical.

       4.  Although the ABNF production 'extlang' permits up to three
           extended language tags in the language tag, extended language
           subtags MUST NOT include another extended language subtag in
           their 'Prefix'.  That is, the second and third extended language
           subtag positions in a language tag are permanently reserved and
           tags that include those subtags in that position are, and will
           always remain, invalid.

       For example, the macrolanguage Chinese ('zh') encompasses a number of
       languages.  For compatibility reasons, each of these languages has
       both a primary and extended language subtag in the registry.  A few
       selected examples of these include Gan Chinese ('gan'), Cantonese
       Chinese ('yue'), and Mandarin Chinese ('cmn').  Each is encompassed
       by the macrolanguage 'zh' (Chinese).  Therefore, they each have the
       prefix "zh" in their registry records.  Thus, Gan Chinese is
       represented with tags beginning "zh-gan" or "gan", Cantonese with
       tags beginning either "yue" or "zh-yue", and Mandarin Chinese with
       "zh-cmn" or "cmn".  The language subtag 'zh' can still be used
       without an extended language subtag to label a resource as some
       unspecified variety of Chinese, while the primary language subtag
       ('gan', 'yue', 'cmn') is preferred to using the extended language
       form ("zh-gan", "zh-yue", "zh-cmn").

    2.2.3.  Script Subtag

       Script subtags are used to indicate the script or writing system
       variations that distinguish the written forms of a language or its
       dialects.  The following rules apply to the script subtags:

       1.  Script subtags MUST follow any primary and extended language
           subtags and MUST precede any other type of subtag.

       2.  Script subtags consist of four letters and were defined according
           to the assignments found in [ISO15924] ("Information and
           documentation -- Codes for the representation of names of
           scripts"), or subsequently assigned by the ISO 15924 registration
           authority or governing standardization bodies.  Only codes
           assigned by ISO 15924 will be considered for registration.

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       3.  The script subtags 'Qaaa' through 'Qabx' are reserved for private
           use in language tags.  These subtags correspond to codes reserved
           by ISO 15924 for private use.  These codes MAY be used for non-
           registered script values.  Please refer to Section 4.6 for more
           information on private use subtags.

       4.  There MUST be at most one script subtag in a language tag, and
           the script subtag SHOULD be omitted when it adds no
           distinguishing value to the tag or when the primary or extended
           language subtag's record in the subtag registry includes a
           'Suppress-Script' field listing the applicable script subtag.

       For example: "sr-Latn" represents Serbian written using the Latin
       script.

    2.2.4.  Region Subtag

       Region subtags are used to indicate linguistic variations associated
       with or appropriate to a specific country, territory, or region.
       Typically, a region subtag is used to indicate variations such as
       regional dialects or usage, or region-specific spelling conventions.
       It can also be used to indicate that content is expressed in a way
       that is appropriate for use throughout a region, for instance,
       Spanish content tailored to be useful throughout Latin America.

       The following rules apply to the region subtags:

       1.  Region subtags MUST follow any primary language, extended
           language, or script subtags and MUST precede any other type of
           subtag.

       2.  Two-letter region subtags were defined according to the
           assignments found in [ISO3166-1] ("Codes for the representation
           of names of countries and their subdivisions -- Part 1: Country
           codes"), using the list of alpha-2 country codes or using
           assignments subsequently made by the ISO 3166-1 maintenance
           agency or governing standardization bodies.  In addition, the
           codes that are "exceptionally reserved" (as opposed to
           "assigned") in ISO 3166-1 were also defined in the registry, with
           the exception of 'UK', which is an exact synonym for the assigned
           code 'GB'.

       3.  The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ' are
           reserved for private use in language tags.  These subtags
           correspond to codes reserved by ISO 3166 for private use.  These
           codes MAY be used for private use region subtags (instead of
           using a private use subtag sequence).  Please refer to
           Section 4.6 for more information on private use subtags.

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       4.  Three-character region subtags consist solely of digit (number)
           characters and were defined according to the assignments found in
           the UN Standard Country or Area Codes for Statistical  Use
           [UN_M.49] or assignments subsequently made by the governing
           standards body.  Not all of the UN M.49 codes are defined in the
           IANA registry.  The following rules define which codes are
           entered into the registry as valid subtags:

           A.  UN numeric codes assigned to 'macro-geographical
               (continental)' or sub-regions MUST be registered in the
               registry.  These codes are not associated with an assigned
               ISO 3166-1 alpha-2 code and represent supra-national areas,
               usually covering more than one nation, state, province, or
               territory.

           B.  UN numeric codes for 'economic groupings' or 'other
               groupings' MUST NOT be registered in the IANA registry and
               MUST NOT be used to form language tags.

           C.  When ISO 3166-1 reassigns a code formerly used for one
               country or area to another country or area and that code
               already is present in the registry, the UN numeric code for
               that country or area MUST be registered in the registry as
               described in Section 3.4 and MUST be used to form language
               tags that represent the country or region for which it is
               defined (rather than the recycled ISO 3166-1 code).

           D.  UN numeric codes for countries or areas for which there is an
               associated ISO 3166-1 alpha-2 code in the registry MUST NOT
               be entered into the registry and MUST NOT be used to form
               language tags.  Note that the ISO 3166-based subtag in the
               registry MUST actually be associated with the UN M.49 code in
               question.

           E.  For historical reasons, the UN numeric code 830 (Channel
               Islands), which was not registered at the time this document
               was adopted and had, at that time, no corresponding ISO
               3166-1 code, MAY be entered into the IANA registry via the
               process described in Section 3.5, provided no ISO 3166-1 code
               with that exact meaning has been previously registered.

           F.  All other UN numeric codes for countries or areas that do not
               have an associated ISO 3166-1 alpha-2 code MUST NOT be
               entered into the registry and MUST NOT be used to form
               language tags.  For more information about these codes, see
               Section 3.4.

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       5.  The alphanumeric codes in Appendix X of the UN document MUST NOT
           be entered into the registry and MUST NOT be used to form
           language tags.  (At the time this document was created, these
           values matched the ISO 3166-1 alpha-2 codes.)

       6.  There MUST be at most one region subtag in a language tag and the
           region subtag MAY be omitted, as when it adds no distinguishing
           value to the tag.

       For example:

          "de-AT" represents German ('de') as used in Austria ('AT').

          "sr-Latn-RS" represents Serbian ('sr') written using Latin script
          ('Latn') as used in Serbia ('RS').

          "es-419" represents Spanish ('es') appropriate to the UN-defined
          Latin America and Caribbean region ('419').

    2.2.5.  Variant Subtags

       Variant subtags are used to indicate additional, well-recognized
       variations that define a language or its dialects that are not
       covered by other available subtags.  The following rules apply to the
       variant subtags:

       1.  Variant subtags MUST follow any primary language, extended
           language, script, or region subtags and MUST precede any
           extension or private use subtag sequences.

       2.  Variant subtags, as a collection, are not associated with any
           particular external standard.  The meaning of variant subtags in
           the registry is defined in the course of the registration process
           defined in Section 3.5.  Note that any particular variant subtag
           might be associated with some external standard.  However,
           association with a standard is not required for registration.

       3.  More than one variant MAY be used to form the language tag.

       4.  Variant subtags MUST be registered with IANA according to the
           rules in Section 3.5 of this document before being used to form
           language tags.  In order to distinguish variants from other types
           of subtags, registrations MUST meet the following length and
           content restrictions:

           1.  Variant subtags that begin with a letter (a-z, A-Z) MUST be
               at least five characters long.

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           2.  Variant subtags that begin with a digit (0-9) MUST be at
               least four characters long.

       5.  The same variant subtag MUST NOT be used more than once within a
           language tag.

           *  For example, the tag "de-DE-1901-1901" is not valid.

       Variant subtag records in the Language Subtag Registry MAY include
       one or more 'Prefix' (Section 3.1.8) fields.  Each 'Prefix' indicates
       a suitable sequence of subtags for forming (with other subtags, as
       appropriate) a language tag when using the variant.

       Most variants that share a prefix are mutually exclusive.  For
       example, the German orthographic variations '1996' and '1901' SHOULD
       NOT be used in the same tag, as they represent the dates of different
       spelling reforms.  A variant that can meaningfully be used in
       combination with another variant SHOULD include a 'Prefix' field in
       its registry record that lists that other variant.  For example, if
       another German variant 'example' were created that made sense to use
       with '1996', then 'example' should include two 'Prefix' fields: "de"
       and "de-1996".

       For example:

          "sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.

          "de-CH-1996" represents German as used in Switzerland and as
          written using the spelling reform beginning in the year 1996 C.E.

    2.2.6.  Extension Subtags

       Extensions provide a mechanism for extending language tags for use in
       various applications.  They are intended to identify information that
       is commonly used in association with languages or language tags but
       that is not part of language identification.  See Section 3.7.  The
       following rules apply to extensions:

       1.  An extension MUST follow at least a primary language subtag.
           That is, a language tag cannot begin with an extension.
           Extensions extend language tags, they do not override or replace
           them.  For example, "a-value" is not a well-formed language tag,
           while "de-a-value" is.  Note that extensions cannot be used in
           tags that are entirely private use (that is, tags starting with
           "x-").

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       2.  Extension subtags are separated from the other subtags defined in
           this document by a single-character subtag (called a
           "singleton").  The singleton MUST be one allocated to a
           registration authority via the mechanism described in Section 3.7
           and MUST NOT be the letter 'x', which is reserved for private use
           subtag sequences.

       3.  Each singleton subtag MUST appear at most one time in each tag
           (other than as a private use subtag).  That is, singleton subtags
           MUST NOT be repeated.  For example, the tag "en-a-bbb-a-ccc" is
           invalid because the subtag 'a' appears twice.  Note that the tag
           "en-a-bbb-x-a-ccc" is valid because the second appearance of the
           singleton 'a' is in a private use sequence.

       4.  Extension subtags MUST meet whatever requirements are set by the
           document that defines their singleton prefix and whatever
           requirements are provided by the maintaining authority.  Note
           that there might not be a registry of these subtags and
           validating processors are not required to validate extensions.

       5.  Each extension subtag MUST be from two to eight characters long
           and consist solely of letters or digits, with each subtag
           separated by a single '-'.  Case distinctions are ignored in
           extensions (as with any language subtag) and normalized subtags
           of this type are expected to be in lowercase.

       6.  Each singleton MUST be followed by at least one extension subtag.
           For example, the tag "tlh-a-b-foo" is invalid because the first
           singleton 'a' is followed immediately by another singleton 'b'.

       7.  Extension subtags MUST follow all primary language, extended
           language, script, region, and variant subtags in a tag and MUST
           precede any private use subtag sequences.

       8.  All subtags following the singleton and before another singleton
           are part of the extension.  Example: In the tag "fr-a-Latn", the
           subtag 'Latn' does not represent the script subtag 'Latn' defined
           in the IANA Language Subtag Registry.  Its meaning is defined by
           the extension 'a'.

       9.  In the event that more than one extension appears in a single
           tag, the tag SHOULD be canonicalized as described in Section 4.5,
           by ordering the various extension sequences into case-insensitive
           ASCII order.

       For example, if an extension were defined for the singleton 'r' and
       it defined the subtags shown, then the following tag would be a valid
       example: "en-Latn-GB-boont-r-extended-sequence-x-private".

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    2.2.7.  Private Use Subtags

       Private use subtags are used to indicate distinctions in language
       that are important in a given context by private agreement.  The
       following rules apply to private use subtags:

       1.  Private use subtags are separated from the other subtags defined
           in this document by the reserved single-character subtag 'x'.

       2.  Private use subtags MUST conform to the format and content
           constraints defined in the ABNF for all subtags; that is, they
           MUST consist solely of letters and digits and not exceed eight
           characters in length.

       3.  Private use subtags MUST follow all primary language, extended
           language, script, region, variant, and extension subtags in the
           tag.  Another way of saying this is that all subtags following
           the singleton 'x' MUST be considered private use.  Example: The
           subtag 'US' in the tag "en-x-US" is a private use subtag.

       4.  A tag MAY consist entirely of private use subtags.

       5.  No source is defined for private use subtags.  Use of private use
           subtags is by private agreement only.

       6.  Private use subtags are NOT RECOMMENDED where alternatives exist
           or for general interchange.  See Section 4.6 for more information
           on private use subtag choice.

       For example, suppose a group of scholars is studying some texts in
       medieval Greek.  They might agree to use some collection of private
       use subtags to identify different styles of writing in the texts.
       For example, they might use 'el-x-koine' for documents in the
       "common" style while using 'el-x-attic' for other documents that
       mimic the Attic style.  These subtags would not be recognized by
       outside processes or systems, but might be useful in categorizing
       various texts for study by those in the group.

       In the registry, there are also subtags derived from codes reserved
       by ISO 639, ISO 15924, or ISO 3166 for private use.  Do not confuse
       these with private use subtag sequences following the subtag 'x'.
       See Section 4.6.

    2.2.8.  Grandfathered and Redundant Registrations

       Prior to RFC 4646, whole language tags were registered according to
       the rules in RFC 1766 and/or RFC 3066.  All of these registered tags
       remain valid as language tags.

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       Many of these registered tags were made redundant by the advent of
       either RFC 4646 or this document.  A redundant tag is a grandfathered
       registration whose individual subtags appear with the same semantic
       meaning in the registry.  For example, the tag "zh-Hant" (Traditional
       Chinese) can now be composed from the subtags 'zh' (Chinese) and
       'Hant' (Han script traditional variant).  These redundant tags are
       maintained in the registry as records of type 'redundant', mostly as
       a matter of historical curiosity.

       The remainder of the previously registered tags are "grandfathered".
       These tags are classified into two groups: 'regular' and 'irregular'.

       Grandfathered tags that (appear to) match the 'langtag' production in
       Figure 1 are considered 'regular' grandfathered tags.  These tags
       contain one or more subtags that either do not individually appear in
       the registry or appear but with a different semantic meaning: each
       tag, in its entirety, represents a language or collection of
       languages.

       Grandfathered tags that do not match the 'langtag' production in the
       ABNF and would otherwise be invalid are considered 'irregular'
       grandfathered tags.  With the exception of "en-GB-oed", which is a
       variant of "en-GB", each of them, in its entirety, represents a
       language.

       Many of the grandfathered tags have been superseded by the subsequent
       addition of new subtags: each superseded record contains a
       'Preferred-Value' field that ought to be used to form language tags
       representing that value.  For example, the tag "art-lojban" is
       superseded by the primary language subtag 'jbo'.

    2.2.9.  Classes of Conformance

       Implementations sometimes need to describe their capabilities with
       regard to the rules and practices described in this document.  Tags
       can be checked or verified in a number of ways, but two particular
       classes of tag conformance are formally defined here.

       A tag is considered "well-formed" if it conforms to the ABNF
       (Section 2.1).  Language tags may be well-formed in terms of syntax
       but not valid in terms of content.  However, many operations
       involving language tags work well without knowing anything about the
       meaning or validity of the subtags.

       A tag is considered "valid" if it satisfies these conditions:

       o  The tag is well-formed.

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       o  Either the tag is in the list of grandfathered tags or all of its
          primary language, extended language, script, region, and variant
          subtags appear in the IANA Language Subtag Registry as of the
          particular registry date.

       o  There are no duplicate variant subtags.

       o  There are no duplicate singleton (extension) subtags.

       Note that a tag's validity depends on the date of the registry used
       to validate the tag.  A more recent copy of the registry might
       contain a subtag that an older version does not.

       A tag is considered valid for a given extension (Section 3.7) (as of
       a particular version, revision, and date) if it meets the criteria
       for "valid" above and also satisfies this condition:

          Each subtag used in the extension part of the tag is valid
          according to the extension.

       Older specifications or language tag implementations sometimes
       reference [RFC3066].  A wider array of tags was considered well-
       formed under that document.  Any tags that were valid for use under
       RFC 3066 are both well-formed and valid under this document's syntax;
       only invalid or illegal tags were well-formed under the earlier
       definition but no longer are.  The language tag syntax under RFC 3066
       was:

           obs-language-tag = primary-subtag *( "-" subtag )
           primary-subtag   = 1*8ALPHA
           subtag           = 1*8(ALPHA / DIGIT)

                      Figure 2: RFC 3066 Language Tag Syntax

       Subtags designated for private use as well as private use sequences
       introduced by the 'x' subtag are available for cases in which no
       assigned subtags are available and registration is not a suitable
       option.  For example, one might use a tag such as "no-QQ", where 'QQ'
       is one of a range of private use ISO 3166-1 codes to indicate an
       otherwise undefined region.  Users MUST NOT assign language tags that
       use subtags that do not appear in the registry other than in private
       use sequences (such as the subtag 'personal' in the tag "en-x-
       personal").  Besides not being valid, the user also risks collision
       with a future possible assignment or registrations.

       Note well: although the 'Language-Tag' production appearing in this
       document is functionally equivalent to the one in [RFC4646], it has

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       been changed to prevent certain errors in well-formedness arising
       from the old 'grandfathered' production.

    3.  Registry Format and Maintenance

       The IANA Language Subtag Registry ("the registry") contains a
       comprehensive list of all of the subtags valid in language tags.
       This allows implementers a straightforward and reliable way to
       validate language tags.  The registry will be maintained so that,
       except for extension subtags, it is possible to validate all of the
       subtags that appear in a language tag under the provisions of this
       document or its revisions or successors.  In addition, the meaning of
       the various subtags will be unambiguous and stable over time.  (The
       meaning of private use subtags, of course, is not defined by the
       registry.)

       This section defines the registry along with the maintenance and
       update procedures associated with it, as well as a registry for
       extensions to language tags (Section 3.7).

    3.1.  Format of the IANA Language Subtag Registry

       The IANA Language Subtag Registry is a machine-readable file in the
       format described in this section, plus copies of the registration
       forms approved in accordance with the process described in
       Section 3.5.

       The existing registration forms for grandfathered and redundant tags
       taken from RFC 3066 have been maintained as part of the obsolete RFC
       3066 registry.  The subtags added to the registry by either [RFC4645]
       or [RFC5645] do not have separate registration forms (so no forms are
       archived for these additions).

    3.1.1.  File Format

       The registry is a [Unicode] text file and consists of a series of
       records in a format based on "record-jar" (described in
       [record-jar]).  Each record, in turn, consists of a series of fields
       that describe the various subtags and tags.  The actual registry file
       is encoded using the UTF-8 [RFC3629] character encoding.

       Each field can be considered a single, logical line of characters.
       Each field contains a "field-name" and a "field-body".  These are
       separated by a "field-separator".  The field-separator is a COLON
       character (U+003A) plus any surrounding whitespace.  Each field is
       terminated by the newline sequence CRLF.  The text in each field MUST
       be in Unicode Normalization Form C (NFC).

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       A collection of fields forms a "record".  Records are separated by
       lines containing only the sequence "%%" (U+0025 U+0025).

       Although fields are logically a single line of text, each line of
       text in the file format is limited to 72 bytes in length.  To
       accommodate this, the field-body can be split into a multiple-line
       representation; this is called "folding".  Folding is done according
       to customary conventions for line-wrapping.  This is typically on
       whitespace boundaries, but can occur between other characters when
       the value does not include spaces, such as when a language does not
       use whitespace between words.  In any event, there MUST NOT be breaks
       inside a multibyte UTF-8 sequence or in the middle of a combining
       character sequence.  For more information, see [UAX14].

       Although the file format uses the Unicode character set and the file
       itself is encoded using the UTF-8 encoding, fields are restricted to
       the printable characters from the US-ASCII [ISO646] repertoire unless
       otherwise indicated in the description of a specific field
       (Section 3.1.2).

       The format of the registry is described by the following ABNF
       [RFC5234].  Character numbers (code points) are taken from Unicode,
       and terminals in the ABNF productions are in terms of characters
       rather than bytes.

       registry   = record *("%%" CRLF record)
       record     = 1*field
       field      = ( field-name field-sep field-body CRLF )
       field-name = (ALPHA / DIGIT) [*(ALPHA / DIGIT / "-") (ALPHA / DIGIT)]
       field-sep  = *SP ":" *SP
       field-body = *([[*SP CRLF] 1*SP] 1*CHARS)
       CHARS      = (%x21-10FFFF)      ; Unicode code points

                          Figure 3: Registry Format ABNF

       The sequence '..'  (U+002E U+002E) in a field-body denotes a range of
       values.  Such a range represents all subtags of the same length that
       are in alphabetic or numeric order within that range, including the
       values explicitly mentioned.  For example, 'a..c' denotes the values
       'a', 'b', and 'c', and '11..13' denotes the values '11', '12', and
       '13'.

       All fields whose field-body contains a date value use the "full-date"
       format specified in [RFC3339].  For example, "2004-06-28" represents
       June 28, 2004, in the Gregorian calendar.

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    3.1.2.  Record and Field Definitions

       There are three types of records in the registry: "File-Date",
       "Subtag", and "Tag".

       The first record in the registry is always the "File-Date" record.
       This record occurs only once in the file and contains a single field
       whose field-name is "File-Date".  The field-body of this record
       contains a date (see Section 5.1), making it possible to easily
       recognize different versions of the registry.

       File-Date: 2004-06-28
       %%

                     Figure 4: Example of the File-Date Record

       Subsequent records contain multiple fields and represent information
       about either subtags or tags.  Both types of records have an
       identical structure, except that "Subtag" records contain a field
       with a field-name of "Subtag", while, unsurprisingly, "Tag" records
       contain a field with a field-name of "Tag".  Field-names MUST NOT
       occur more than once per record, with the exception of the
       'Description', 'Comments', and 'Prefix' fields.

       Each record MUST contain at least one of each of the following
       fields:

       o  'Type'

          *  Type's field-body MUST consist of one of the following strings:
             "language", "extlang", "script", "region", "variant",
             "grandfathered", and "redundant"; it denotes the type of tag or
             subtag.

       o  Either 'Subtag' or 'Tag'

          *  Subtag's field-body contains the subtag being defined.  This
             field MUST appear in all records whose 'Type' has one of these
             values: "language", "extlang", "script", "region", or
             "variant".

          *  Tag's field-body contains a complete language tag.  This field
             MUST appear in all records whose 'Type' has one of these
             values: "grandfathered" or "redundant".  If the 'Type' is
             "grandfathered", then the 'Tag' field-body will be one of the
             tags listed in either the 'regular' or 'irregular' production
             found in Section 2.1.

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       o  'Description'

          *  Description's field-body contains a non-normative description
             of the subtag or tag.

       o  'Added'

          *  Added's field-body contains the date the record was registered
             or, in the case of grandfathered or redundant tags, the date
             the corresponding tag was registered under the rules of
             [RFC1766] or [RFC3066].

       Each record MAY also contain the following fields:

       o  'Deprecated'

          *  Deprecated's field-body contains the date the record was
             deprecated.  In some cases, this value is earlier than that of
             the 'Added' field in the same record.  That is, the date of
             deprecation preceded the addition of the record to the
             registry.

       o  'Preferred-Value'

          *  Preferred-Value's field-body contains a canonical mapping from
             this record's value to a modern equivalent that is preferred in
             its place.  Depending on the value of the 'Type' field, this
             value can take different forms:

             +  For fields of type 'language', 'Preferred-Value' contains
                the primary language subtag that is preferred when forming
                the language tag.

             +  For fields of type 'script', 'region', or 'variant',
                'Preferred-Value' contains the subtag of the same type that
                is preferred for forming the language tag.

             +  For fields of type 'extlang', 'grandfathered', or
                'redundant', 'Preferred-Value' contains an "extended
                language range" [RFC4647] that is preferred for forming the
                language tag.  That is, the preferred language tag will
                contain, in order, each of the subtags that appears in the
                'Preferred-Value'; additional fields can be included in a
                language tag, as described elsewhere in this document.  For
                example, the replacement for the grandfathered tag "zh-min-
                nan" (Min Nan Chinese) is "nan", which can be used as the

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    RFC 5646                     Language Tags                September 2009

                basis for tags such as "nan-Hant" or "nan-TW" (note that the
                extended language subtag form such as "zh-nan-Hant" or "zh-
                nan-TW" can also be used).

       o  'Prefix'

          *  Prefix's field-body contains a valid language tag that is
             RECOMMENDED as one possible prefix to this record's subtag.
             This field MAY appear in records whose 'Type' field-body is
             either 'extlang' or 'variant' (it MUST NOT appear in any other
             record type).

       o  'Suppress-Script'

          *  Suppress-Script's field-body contains a script subtag that
             SHOULD NOT be used to form language tags with the associated
             primary or extended language subtag.  This field MUST appear
             only in records whose 'Type' field-body is 'language' or
             'extlang'.  See Section 4.1.

       o  'Macrolanguage'

          *  Macrolanguage's field-body contains a primary language subtag
             defined by ISO 639 as the "macrolanguage" that encompasses this
             language subtag.  This field MUST appear only in records whose
             'Type' field-body is either 'language' or 'extlang'.

       o  'Scope'

          *  Scope's field-body contains information about a primary or
             extended language subtag indicating the type of language code
             according to ISO 639.  The values permitted in this field are
             "macrolanguage", "collection", "special", and "private-use".
             This field only appears in records whose 'Type' field-body is
             either 'language' or 'extlang'.  When this field is omitted,
             the language is an individual language.

       o  'Comments'

          *  Comments's field-body contains additional information about the
             subtag, as deemed appropriate for understanding the registry
             and implementing language tags using the subtag or tag.

       Future versions of this document might add additional fields to the
       registry; implementations SHOULD ignore fields found in the registry
       that are not defined in this document.

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    3.1.3.  Type Field

       The field 'Type' contains the string identifying the record type in
       which it appears.  Values for the 'Type' field-body are: "language"
       (Section 2.2.1); "extlang" (Section 2.2.2); "script" (Section 2.2.3);
       "region" (Section 2.2.4); "variant" (Section 2.2.5); "grandfathered"
       or "redundant" (Section 2.2.8).

    3.1.4.  Subtag and Tag Fields

       The field 'Subtag' contains the subtag defined in the record.  The
       field 'Tag' appears in records whose 'Type' is either 'grandfathered'
       or 'redundant' and contains a tag registered under [RFC3066].

       The 'Subtag' field-body MUST follow the casing conventions described
       in Section 2.1.1.  All subtags use lowercase letters in the field-
       body, with two exceptions:

          Subtags whose 'Type' field is 'script' (in other words, subtags
          defined by ISO 15924) MUST use titlecase.

          Subtags whose 'Type' field is 'region' (in other words, the non-
          numeric region subtags defined by ISO 3166-1) MUST use all
          uppercase.

       The 'Tag' field-body MUST be formatted according to the rules
       described in Section 2.1.1.

    3.1.5.  Description Field

       The field 'Description' contains a description of the tag or subtag
       in the record.  The 'Description' field MAY appear more than once per
       record.  The 'Description' field MAY include the full range of
       Unicode characters.  At least one of the 'Description' fields MUST be
       written or transcribed into the Latin script; additional
       'Description' fields MAY be in any script or language.

       The 'Description' field is used for identification purposes.
       Descriptions SHOULD contain all and only that information necessary
       to distinguish one subtag from others with which it might be
       confused.  They are not intended to provide general background
       information or to provide all possible alternate names or
       designations.  'Description' fields don't necessarily represent the
       actual native name of the item in the record, nor are any of the
       descriptions guaranteed to be in any particular language (such as
       English or French, for example).

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       Descriptions in the registry that correspond to ISO 639, ISO 15924,
       ISO 3166-1, or UN M.49 codes are intended only to indicate the
       meaning of that identifier as defined in the source standard at the
       time it was added to the registry or as subsequently modified, within
       the bounds of the stability rules (Section 3.4), via subsequent
       registration.  The 'Description' does not replace the content of the
       source standard itself.  'Description' fields are not intended to be
       the localized English names for the subtags.  Localization or
       translation of language tag and subtag descriptions is out of scope
       of this document.

       For subtags taken from a source standard (such as ISO 639 or ISO
       15924), the 'Description' fields in the record are also initially
       taken from that source standard.  Multiple descriptions in the source
       standard are split into separate 'Description' fields.  The source
       standard's descriptions MAY be edited or modified, either prior to
       insertion or via the registration process, and additional or
       extraneous descriptions omitted or removed.  Each 'Description' field
       MUST be unique within the record in which it appears, and formatting
       variations of the same description SHOULD NOT occur in that specific
       record.  For example, while the ISO 639-1 code 'fy' has both the
       description "Western Frisian" and the description "Frisian, Western"
       in that standard, only one of these descriptions appears in the
       registry.

       To help ensure that users do not become confused about which subtag
       to use, 'Description' fields assigned to a record of any specific
       type ('language', 'extlang', 'script', and so on) MUST be unique
       within that given record type with the following exception: if a
       particular 'Description' field occurs in multiple records of a given
       type, then at most one of the records can omit the 'Deprecated'
       field.  All deprecated records that share a 'Description' MUST have
       the same 'Preferred-Value', and all non-deprecated records MUST be
       that 'Preferred-Value'.  This means that two records of the same type
       that share a 'Description' are also semantically equivalent and no
       more than one record with a given 'Description' is preferred for that
       meaning.

       For example, consider the 'language' subtags 'zza' (Zaza) and 'diq'
       (Dimli).  It so happens that 'zza' is a macrolanguage enclosing 'diq'
       and thus also has a description in ISO 639-3 of "Dimli".  This
       description was edited to read "Dimli (macrolanguage)" in the
       registry record for 'zza' to prevent a collision.

       By contrast, the subtags 'he' and 'iw' share a 'Description' value of
       "Hebrew"; this is permitted because 'iw' is deprecated and its
       'Preferred-Value' is 'he'.

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       For fields of type 'language', the first 'Description' field
       appearing in the registry corresponds whenever possible to the
       Reference Name assigned by ISO 639-3.  This helps facilitate cross-
       referencing between ISO 639 and the registry.

       When creating or updating a record due to the action of one of the
       source standards, the Language Subtag Reviewer MAY edit descriptions
       to correct irregularities in formatting (such as misspellings,
       inappropriate apostrophes or other punctuation, or excessive or
       missing spaces) prior to submitting the proposed record to the
       ietf-languages@iana.org list for consideration.

    3.1.6.  Deprecated Field

       The field 'Deprecated' contains the date the record was deprecated
       and MAY be added, changed, or removed from any record via the
       maintenance process described in Section 3.3 or via the registration
       process described in Section 3.5.  Usually, the addition of a
       'Deprecated' field is due to the action of one of the standards
       bodies, such as ISO 3166, withdrawing a code.  Although valid in
       language tags, subtags and tags with a 'Deprecated' field are
       deprecated, and validating processors SHOULD NOT generate these
       subtags.  Note that a record that contains a 'Deprecated' field and
       no corresponding 'Preferred-Value' field has no replacement mapping.

       In some historical cases, it might not have been possible to
       reconstruct the original deprecation date.  For these cases, an
       approximate date appears in the registry.  Some subtags and some
       grandfathered or redundant tags were deprecated before the initial
       creation of the registry.  The exact rules for this appear in Section
       2 of [RFC4645].  Note that these records have a 'Deprecated' field
       with an earlier date then the corresponding 'Added' field!

    3.1.7.  Preferred-Value Field

       The field 'Preferred-Value' contains a mapping between the record in
       which it appears and another tag or subtag (depending on the record's
       'Type').  The value in this field is used for canonicalization (see
       Section 4.5).  In cases where the subtag or tag also has a
       'Deprecated' field, then the 'Preferred-Value' is RECOMMENDED as the
       best choice to represent the value of this record when selecting a
       language tag.

       Records containing a 'Preferred-Value' fall into one of these four
       groups:

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       1.  ISO 639 language codes that were later withdrawn in favor of
           other codes.  These values are mostly a historical curiosity.
           The 'he'/'iw' pairing above is an example of this.

       2.  Subtags (with types other than language or extlang) taken from
           codes or values that have been withdrawn in favor of a new code.
           In particular, this applies to region subtags taken from ISO
           3166-1, because sometimes a country will change its name or
           administration in such a way that warrants a new region code.  In
           some cases, countries have reverted to an older name, which might
           already be encoded.  For example, the subtag 'ZR' (Zaire) was
           replaced by the subtag 'CD' (Democratic Republic of the Congo)
           when that country's name was changed.

       3.  Tags or subtags that have become obsolete because the values they
           represent were later encoded.  Many of the grandfathered or
           redundant tags were later encoded by ISO 639, for example, and
           fall into this grouping.  For example, "i-klingon" was deprecated
           when the subtag 'tlh' was added.  The record for "i-klingon" has
           a 'Preferred-Value' of 'tlh'.

       4.  Extended language subtags always have a mapping to their
           identical primary language subtag.  For example, the extended
           language subtag 'yue' (Cantonese) can be used to form the tag
           "zh-yue".  It has a 'Preferred-Value' mapping to the primary
           language subtag 'yue', meaning that a tag such as
           "zh-yue-Hant-HK" can be canonicalized to "yue-Hant-HK".

       Records other than those of type 'extlang' that contain a 'Preferred-
       Value' field MUST also have a 'Deprecated' field.  This field
       contains the date on which the tag or subtag was deprecated in favor
       of the preferred value.

       For records of type 'extlang', the 'Preferred-Value' field appears
       without a corresponding 'Deprecated' field.  An implementation MAY
       ignore these preferred value mappings, although if it ignores the
       mapping, it SHOULD do so consistently.  It SHOULD also treat the
       'Preferred-Value' as equivalent to the mapped item.  For example, the
       tags "zh-yue-Hant-HK" and "yue-Hant-HK" are semantically equivalent
       and ought to be treated as if they were the same tag.

       Occasionally, the deprecated code is preferred in certain contexts.
       For example, both "iw" and "he" can be used in the Java programming
       language, but "he" is converted on input to "iw", which is thus the
       canonical form in Java.

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       'Preferred-Value' mappings in records of type 'region' sometimes do
       not represent exactly the same meaning as the original value.  There
       are many reasons for a country code to be changed, and the effect
       this has on the formation of language tags will depend on the nature
       of the change in question.  For example, the region subtag 'YD'
       (Democratic Yemen) was deprecated in favor of the subtag 'YE' (Yemen)
       when those two countries unified in 1990.

       A 'Preferred-Value' MAY be added to, changed, or removed from records
       according to the rules in Section 3.3.  Addition, modification, or
       removal of a 'Preferred-Value' field in a record does not imply that
       content using the affected subtag needs to be retagged.

       The 'Preferred-Value' fields in records of type "grandfathered" and
       "redundant" each contain an "extended language range" [RFC4647] that
       is strongly RECOMMENDED for use in place of the record's value.  In
       many cases, these mappings were created via deprecation of the tags
       during the period before [RFC4646] was adopted.  For example, the tag
       "no-nyn" was deprecated in favor of the ISO 639-1-defined language
       code 'nn'.

       The 'Preferred-Value' field in subtag records of type "extlang" also
       contains an "extended language range".  This allows the subtag to be
       deprecated in favor of either a single primary language subtag or a
       new language-extlang sequence.

       Usually, the addition, removal, or change of a 'Preferred-Value'
       field for a subtag is done to reflect changes in one of the source
       standards.  For example, if an ISO 3166-1 region code is deprecated
       in favor of another code, that SHOULD result in the addition of a
       'Preferred-Value' field.

       Changes to one subtag can affect other subtags as well: when
       proposing changes to the registry, the Language Subtag Reviewer MUST
       review the registry for such effects and propose the necessary
       changes using the process in Section 3.5, although anyone MAY request
       such changes.  For example:

          Suppose that subtag 'XX' has a 'Preferred-Value' of 'YY'.  If 'YY'
          later changes to have a 'Preferred-Value' of 'ZZ', then the
          'Preferred-Value' for 'XX' MUST also change to be 'ZZ'.

          Suppose that a registered language subtag 'dialect' represents a
          language not yet available in any part of ISO 639.  The later
          addition of a corresponding language code in ISO 639 SHOULD result
          in the addition of a 'Preferred-Value' for 'dialect'.

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    3.1.8.  Prefix Field

       The field 'Prefix' contains a valid language tag that is RECOMMENDED
       as one possible prefix to this record's subtag, perhaps with other
       subtags.  That is, when including an extended language or a variant
       subtag that has at least one 'Prefix' in a language tag, the
       resulting tag SHOULD match at least one of the subtag's 'Prefix'
       fields using the "Extended Filtering" algorithm (see [RFC4647]), and
       each of the subtags in that 'Prefix' SHOULD appear before the subtag
       itself.

       The 'Prefix' field MUST appear exactly once in a record of type
       'extlang'.  The 'Prefix' field MAY appear multiple times (or not at
       all) in records of type 'variant'.  Additional fields of this type
       MAY be added to a 'variant' record via the registration process,
       provided the 'variant' record already has at least one 'Prefix'
       field.

       Each 'Prefix' field indicates a particular sequence of subtags that
       form a meaningful tag with this subtag.  For example, the extended
       language subtag 'cmn' (Mandarin Chinese) only makes sense with its
       prefix 'zh' (Chinese).  Similarly, 'rozaj' (Resian, a dialect of
       Slovenian) would be appropriate when used with its prefix 'sl'
       (Slovenian), while tags such as "is-1994" are not appropriate (and
       probably not meaningful).  Although the 'Prefix' for 'rozaj' is "sl",
       other subtags might appear between them.  For example, the tag "sl-
       IT-rozaj" (Slovenian, Italy, Resian) matches the 'Prefix' "sl".

       The 'Prefix' also indicates when variant subtags make sense when used
       together (many that otherwise share a 'Prefix' are mutually
       exclusive) and what the relative ordering of variants is supposed to
       be.  For example, the variant '1994' (Standardized Resian
       orthography) has several 'Prefix' fields in the registry ("sl-rozaj",
       "sl-rozaj-biske", "sl-rozaj-njiva", "sl-rozaj-osojs", and "sl-rozaj-
       solba").  This indicates not only that '1994' is appropriate to use
       with each of these five Resian variant subtags ('rozaj', 'biske',
       'njiva', 'osojs', and 'solba'), but also that it SHOULD appear
       following any of these variants in a tag.  Thus, the language tag
       ought to take the form "sl-rozaj-biske-1994", rather than "sl-1994-
       rozaj-biske" or "sl-rozaj-1994-biske".

       If a record includes no 'Prefix' field, a 'Prefix' field MUST NOT be
       added to the record at a later date.  Otherwise, changes (additions,
       deletions, or modifications) to the set of 'Prefix' fields MAY be
       registered, as long as they strictly widen the range of language tags
       that are recommended.  For example, a 'Prefix' with the value "be-
       Latn" (Belarusian, Latin script) could be replaced by the value "be"
       (Belarusian) but not by the value "ru-Latn" (Russian, Latin script)

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       or the value "be-Latn-BY" (Belarusian, Latin script, Belarus), since
       these latter either change or narrow the range of suggested tags.

       The field-body of the 'Prefix' field MUST NOT conflict with any
       'Prefix' already registered for a given record.  Such a conflict
       would occur when no valid tag could be constructed that would contain
       the prefix, such as when two subtags each have a 'Prefix' that
       contains the other subtag.  For example, suppose that the subtag
       'avariant' has the prefix "es-bvariant".  Then the subtag 'bvariant'
       cannot be assigned the prefix 'avariant', for that would require a
       tag of the form "es-avariant-bvariant-avariant", which would not be
       valid.

    3.1.9.  Suppress-Script Field

       The field 'Suppress-Script' contains a script subtag (whose record
       appears in the registry).  The field 'Suppress-Script' MUST appear
       only in records whose 'Type' field-body is either 'language' or
       'extlang'.  This field MUST NOT appear more than one time in a
       record.

       This field indicates a script used to write the overwhelming majority
       of documents for the given language.  The subtag for such a script
       therefore adds no distinguishing information to a language tag and
       thus SHOULD NOT be used for most documents in that language.
       Omitting the script subtag indicated by this field helps ensure
       greater compatibility between the language tags generated according
       to the rules in this document and language tags and tag processors or
       consumers based on RFC 3066.  For example, virtually all Icelandic
       documents are written in the Latin script, making the subtag 'Latn'
       redundant in the tag "is-Latn".

       Many language subtag records do not have a 'Suppress-Script' field.
       The lack of a 'Suppress-Script' might indicate that the language is
       customarily written in more than one script or that the language is
       not customarily written at all.  It might also mean that sufficient
       information was not available when the record was created and thus
       remains a candidate for future registration.

    3.1.10.  Macrolanguage Field

       The field 'Macrolanguage' contains a primary language subtag (whose
       record appears in the registry).  This field indicates a language
       that encompasses this subtag's language according to assignments made
       by ISO 639-3.

       ISO 639-3 labels some languages in the registry as "macrolanguages".
       ISO 639-3 defines the term "macrolanguage" to mean "clusters of

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       closely-related language varieties that [...] can be considered
       distinct individual languages, yet in certain usage contexts a single
       language identity for all is needed".  These correspond to codes
       registered in ISO 639-2 as individual languages that were found to
       correspond to more than one language in ISO 639-3.

       A language contained within a macrolanguage is called an "encompassed
       language".  The record for each encompassed language contains a
       'Macrolanguage' field in the registry; the macrolanguages themselves
       are not specially marked.  Note that some encompassed languages have
       ISO 639-1 or ISO 639-2 codes.

       The 'Macrolanguage' field can only occur in records of type
       'language' or 'extlang'.  Only values assigned by ISO 639-3 will be
       considered for inclusion.  'Macrolanguage' fields MAY be added or
       removed via the normal registration process whenever ISO 639-3
       defines new values or withdraws old values.  Macrolanguages are
       informational, and MAY be removed or changed if ISO 639-3 changes the
       values.  For more information on the use of this field and choosing
       between macrolanguage and encompassed language subtags, see
       Section 4.1.1.

       For example, the language subtags 'nb' (Norwegian Bokmal) and 'nn'
       (Norwegian Nynorsk) each have a 'Macrolanguage' field with a value of
       'no' (Norwegian).  For more information, see Section 4.1.

    3.1.11.  Scope Field

       The field 'Scope' contains classification information about a primary
       or extended language subtag derived from ISO 639.  Most languages
       have a scope of 'individual', which means that the language is not a
       macrolanguage, collection, special code, or private use.  That is, it
       is what one would normally consider to be 'a language'.  Any primary
       or extended language subtag that has no 'Scope' field is an
       individual language.

       'Scope' information can sometimes be helpful in selecting language
       tags, since it indicates the purpose or "scope" of the code
       assignment within ISO 639.  The available values are:

       o  'macrolanguage' - Indicates a macrolanguage as defined by ISO
          639-3 (see Section 3.1.10).  A macrolanguage is a cluster of
          closely related languages that are sometimes considered to be a
          single language.

       o  'collection' - Indicates a subtag that represents a collection of
          languages, typically related by some type of historical,
          geographical, or linguistic association.  Unlike a macrolanguage,

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          a collection can contain languages that are only loosely related
          and a collection cannot be used interchangeably with languages
          that belong to it.

       o  'special' - Indicates a special language code.  These are subtags
          used for identifying linguistic attributes not particularly
          associated with a concrete language.  These include codes for when
          the language is undetermined or for non-linguistic content.

       o  'private-use' - Indicates a code reserved for private use in the
          underlying standard.  Subtags with this scope can be used to
          indicate a primary language for which no ISO 639 or registered
          assignment exists.

       The 'Scope' field MAY appear in records of type 'language' or
       'extlang'.  Note that many of the prefixes for extended language
       subtags will have a 'Scope' of 'macrolanguage' (although some will
       not) and that many languages that have a 'Scope' of 'macrolanguage'
       will have extended language subtags associated with them.

       The 'Scope' field MAY be added, modified, or removed via the
       registration process, provided the change mirrors changes made by ISO
       639 to the assignment's classification.  Such a change is expected to
       be rare.

       For example, the primary language subtag 'zh' (Chinese) has a 'Scope'
       of 'macrolanguage', while its enclosed language 'nan' (Min Nan
       Chinese) has a 'Scope' of 'individual'.  The special value 'und'
       (Undetermined) has a 'Scope' of 'special'.  The ISO 639-5 collection
       'gem' (Germanic languages) has a 'Scope' of 'collection'.

    3.1.12.  Comments Field

       The field 'Comments' contains additional information about the record
       and MAY appear more than once per record.  The field-body MAY include
       the full range of Unicode characters and is not restricted to any
       particular script.  This field MAY be inserted or changed via the
       registration process, and no guarantee of stability is provided.

       The content of this field is not restricted, except by the need to
       register the information, the suitability of the request, and by
       reasonable practical size limitations.  The primary reason for the
       'Comments' field is subtag identification -- to help distinguish the
       subtag from others with which it might be confused as an aid to
       usage.  Large amounts of information about the use, history, or
       general background of a subtag are frowned upon, as these generally
       belong in a registration request rather than in the registry.

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    3.2.  Language Subtag Reviewer

       The Language Subtag Reviewer moderates the ietf-languages@iana.org
       mailing list, responds to requests for registration, and performs the
       other registry maintenance duties described in Section 3.3.  Only the
       Language Subtag Reviewer is permitted to request IANA to change,
       update, or add records to the Language Subtag Registry.  The Language
       Subtag Reviewer MAY delegate list moderation and other clerical
       duties as needed.

       The Language Subtag Reviewer is appointed by the IESG for an
       indefinite term, subject to removal or replacement at the IESG's
       discretion.  The IESG will solicit nominees for the position (upon
       adoption of this document or upon a vacancy) and then solicit
       feedback on the nominees' qualifications.  Qualified candidates
       should be familiar with BCP 47 and its requirements; be willing to
       fairly, responsively, and judiciously administer the registration
       process; and be suitably informed about the issues of language
       identification so that the reviewer can assess the claims and draw
       upon the contributions of language experts and subtag requesters.

       The subsequent performance or decisions of the Language Subtag
       Reviewer MAY be appealed to the IESG under the same rules as other
       IETF decisions (see [RFC2026]).  The IESG can reverse or overturn the
       decisions of the Language Subtag Reviewer, provide guidance, or take
       other appropriate actions.

    3.3.  Maintenance of the Registry

       Maintenance of the registry requires that, as codes are assigned or
       withdrawn by ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language
       Subtag Reviewer MUST evaluate each change and determine the
       appropriate course of action according to the rules in this document.
       Such updates follow the registration process described in
       Section 3.5.  Usually, the Language Subtag Reviewer will start the
       process for the new or updated record by filling in the registration
       form and submitting it.  If a change to one of these standards takes
       place and the Language Subtag Reviewer does not do this in a timely
       manner, then any interested party MAY submit the form.  Thereafter,
       the registration process continues normally.

       Note that some registrations affect other subtags--perhaps more than
       one--as when a region subtag is being deprecated in favor of a new
       value.  The Language Subtag Reviewer is responsible for ensuring that
       any such changes are properly registered, with each change requiring
       its own registration form.

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       The Language Subtag Reviewer MUST ensure that new subtags meet the
       requirements elsewhere in this document (and most especially in
       Section 3.4) or submit an appropriate registration form for an
       alternate subtag as described in that section.  Each individual
       subtag affected by a change MUST be sent to the
       ietf-languages@iana.org list with its own registration form and in a
       separate message.

    3.4.  Stability of IANA Registry Entries

       The stability of entries and their meaning in the registry is
       critical to the long-term stability of language tags.  The rules in
       this section guarantee that a specific language tag's meaning is
       stable over time and will not change.

       These rules specifically deal with how changes to codes (including
       withdrawal and deprecation of codes) maintained by ISO 639, ISO
       15924, ISO 3166, and UN M.49 are reflected in the IANA Language
       Subtag Registry.  Assignments to the IANA Language Subtag Registry
       MUST follow the following stability rules:

       1.   Values in the fields 'Type', 'Subtag', 'Tag', and 'Added' MUST
            NOT be changed and are guaranteed to be stable over time.

       2.   Values in the fields 'Preferred-Value' and 'Deprecated' MAY be
            added, altered, or removed via the registration process.  These
            changes SHOULD be limited to changes necessary to mirror changes
            in one of the underlying standards (ISO 639, ISO 15924, ISO
            3166-1, or UN M.49) and typically alteration or removal of a
            'Preferred-Value' is limited specifically to region codes.

       3.   Values in the 'Description' field MUST NOT be changed in a way
            that would invalidate any existing tags.  The description MAY be
            broadened somewhat in scope, changed to add information, or
            adapted to the most common modern usage.  For example, countries
            occasionally change their names; a historical example of this is
            "Upper Volta" changing to "Burkina Faso".

       4.   Values in the field 'Prefix' MAY be added to existing records of
            type 'variant' via the registration process, provided the
            'variant' already has at least one 'Prefix'.  A 'Prefix' field
            SHALL NOT be registered for any 'variant' that has no existing
            'Prefix' field.  If a prefix is added to a variant record,
            'Comment' fields MAY be used to explain different usages with
            the various prefixes.

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       5.   Values in the field 'Prefix' in records of type 'variant' MAY
            also be modified, so long as the modifications broaden the set
            of prefixes.  That is, a prefix MAY be replaced by one of its
            own prefixes.  For example, the prefix "en-US" could be replaced
            by "en", but not by the prefixes "en-Latn", "fr", or "en-US-
            boont".  If one of those prefix values were needed, it would
            have to be separately registered.

       6.   Values in the field 'Prefix' in records of type 'extlang' MUST
            NOT be added, modified, or removed.

       7.   The field 'Prefix' MUST NOT be removed from any record in which
            it appears.  This field SHOULD be included in the initial
            registration of any records of type 'variant' and MUST be
            included in any records of type 'extlang'.

       8.   The field 'Comments' MAY be added, changed, modified, or removed
            via the registration process or any of the processes or
            considerations described in this section.

       9.   The field 'Suppress-Script' MAY be added or removed via the
            registration process.

       10.  The field 'Macrolanguage' MAY be added or removed via the
            registration process, but only in response to changes made by
            ISO 639.  The 'Macrolanguage' field appears whenever a language
            has a corresponding macrolanguage in ISO 639.  That is, the
            'Macrolanguage' fields in the registry exactly match those of
            ISO 639.  No other macrolanguage mappings will be considered for
            registration.

       11.  The field 'Scope' MAY be added or removed from a primary or
            extended language subtag after initial registration, and it MAY
            be modified in order to match any changes made by ISO 639.
            Changes to the 'Scope' field MUST mirror changes made by ISO
            639.  Note that primary or extended language subtags whose
            records do not contain a 'Scope' field (that is, most of them)
            are individual languages as described in Section 3.1.11.

       12.  Primary and extended language subtags (other than independently
            registered values created using the registration process) are
            created according to the assignments of the various parts of ISO
            639, as follows:

            A.  Codes assigned by ISO 639-1 that do not conflict with
                existing two-letter primary language subtags and that have
                no corresponding three-letter primary defined in the
                registry are entered into the IANA registry as new records

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                of type 'language'.  Note that languages given an ISO 639-1
                code cannot be given extended language subtags, even if
                encompassed by a macrolanguage.

            B.  Codes assigned by ISO 639-3 or ISO 639-5 that do not
                conflict with existing three-letter primary language subtags
                and that do not have ISO 639-1 codes assigned (or expected
                to be assigned) are entered into the IANA registry as new
                records of type 'language'.  Note that these two standards
                now comprise a superset of ISO 639-2 codes.  Codes that have
                a defined 'macrolanguage' mapping at the time of their
                registration MUST contain a 'Macrolanguage' field.

            C.  Codes assigned by ISO 639-3 MAY also be considered for an
                extended language subtag registration.  Note that they MUST
                be assigned a primary language subtag record of type
                'language' even when an 'extlang' record is proposed.  When
                considering extended language subtag assignment, these
                criteria apply:

                1.  If a language has a macrolanguage mapping, and that
                    macrolanguage has other encompassed languages that are
                    assigned extended language subtags, then the new
                    language SHOULD have an 'extlang' record assigned to it
                    as well.  For example, any language with a macrolanguage
                    of 'zh' or 'ar' would be assigned an 'extlang' record.

                2.  'Extlang' records SHOULD NOT be created for languages if
                    other languages encompassed by the macrolanguage do not
                    also include 'extlang' records.  For example, if a new
                    Serbo-Croatian ('sh') language were registered, it would
                    not get an extlang record because other languages
                    encompassed, such as Serbian ('sr'), do not include one
                    in the registry.

                3.  Sign languages SHOULD have an 'extlang' record with a
                    'Prefix' of 'sgn'.

                4.  'Extlang' records MUST NOT be created for items already
                    in the registry.  Extended language subtags will only be
                    considered at the time of initial registration.

                5.  Extended language subtag records MUST include the fields
                    'Prefix' and 'Preferred-Value' with field values
                    assigned as described in Section 2.2.2.

            D.  Any other codes assigned by ISO 639-2 that do not conflict
                with existing three-letter primary or extended language

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                subtags and that do not have ISO 639-1 two-letter codes
                assigned are entered into the IANA registry as new records
                of type 'language'.  This type of registration is not
                supposed to occur in the future.

       13.  Codes assigned by ISO 15924 and ISO 3166-1 that do not conflict
            with existing subtags of the associated type and whose meaning
            is not the same as an existing subtag of the same type are
            entered into the IANA registry as new records.

       14.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that are
            withdrawn by their respective maintenance or registration
            authority remain valid in language tags.  A 'Deprecated' field
            containing the date of withdrawal MUST be added to the record.
            If a new record of the same type is added that represents a
            replacement value, then a 'Preferred-Value' field MAY also be
            added.  The registration process MAY be used to add comments
            about the withdrawal of the code by the respective standard.

               For example: the region code 'TL' was assigned to the country
               'Timor-Leste', replacing the code 'TP' (which was assigned to
               'East Timor' when it was under administration by Portugal).
               The subtag 'TP' remains valid in language tags, but its
               record contains the 'Preferred-Value' of 'TL' and its field
               'Deprecated' contains the date the new code was assigned
               ('2004-07-06').

       15.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that
            conflict with existing subtags of the associated type, including
            subtags that are deprecated, MUST NOT be entered into the
            registry.  The following additional considerations apply to
            subtag values that are reassigned:

            A.  For ISO 639 codes, if the newly assigned code's meaning is
                not represented by a subtag in the IANA registry, the
                Language Subtag Reviewer, as described in Section 3.5, SHALL
                prepare a proposal for entering in the IANA registry, as
                soon as practical, a registered language subtag as an
                alternate value for the new code.  The form of the
                registered language subtag will be at the discretion of the
                Language Subtag Reviewer and MUST conform to other
                restrictions on language subtags in this document.

            B.  For all subtags whose meaning is derived from an external
                standard (that is, by ISO 639, ISO 15924, ISO 3166-1, or UN
                M.49), if a new meaning is assigned to an existing code and
                the new meaning broadens the meaning of that code, then the
                meaning for the associated subtag MAY be changed to match.

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                The meaning of a subtag MUST NOT be narrowed, however, as
                this can result in an unknown proportion of the existing
                uses of a subtag becoming invalid.  Note: the ISO 639
                registration authority (RA) has adopted a similar stability
                policy.

            C.  For ISO 15924 codes, if the newly assigned code's meaning is
                not represented by a subtag in the IANA registry, the
                Language Subtag Reviewer, as described in Section 3.5, SHALL
                prepare a proposal for entering in the IANA registry, as
                soon as practical, a registered variant subtag as an
                alternate value for the new code.  The form of the
                registered variant subtag will be at the discretion of the
                Language Subtag Reviewer and MUST conform to other
                restrictions on variant subtags in this document.

            D.  For ISO 3166-1 codes, if the newly assigned code's meaning
                is associated with the same UN M.49 code as another 'region'
                subtag, then the existing region subtag remains as the
                preferred value for that region and no new entry is created.
                A comment MAY be added to the existing region subtag
                indicating the relationship to the new ISO 3166-1 code.

            E.  For ISO 3166-1 codes, if the newly assigned code's meaning
                is associated with a UN M.49 code that is not represented by
                an existing region subtag, then the Language Subtag
                Reviewer, as described in Section 3.5, SHALL prepare a
                proposal for entering the appropriate UN M.49 country code
                as an entry in the IANA registry.

            F.  For ISO 3166-1 codes, if there is no associated UN numeric
                code, then the Language Subtag Reviewer SHALL petition the
                UN to create one.  If there is no response from the UN
                within 90 days of the request being sent, the Language
                Subtag Reviewer SHALL prepare a proposal for entering in the
                IANA registry, as soon as practical, a registered variant
                subtag as an alternate value for the new code.  The form of
                the registered variant subtag will be at the discretion of
                the Language Subtag Reviewer and MUST conform to other
                restrictions on variant subtags in this document.  This
                situation is very unlikely to ever occur.

       16.  UN M.49 has codes for both "countries and areas" (such as '276'
            for Germany) and "geographical regions and sub-regions" (such as
            '150' for Europe).  UN M.49 country or area codes for which
            there is no corresponding ISO 3166-1 code MUST NOT be
            registered, except as a surrogate for an ISO 3166-1 code that is
            blocked from registration by an existing subtag.

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            If such a code becomes necessary, then the maintenance agency
            for ISO 3166-1 SHALL first be petitioned to assign a code to the
            region.  If the petition for a code assignment by ISO 3166-1 is
            refused or not acted on in a timely manner, the registration
            process described in Section 3.5 can then be used to register
            the corresponding UN M.49 code.  This way, UN M.49 codes remain
            available as the value of last resort in cases where ISO 3166-1
            reassigns a deprecated value in the registry.

       17.  The redundant and grandfathered entries together form the
            complete list of tags registered under [RFC3066].  The redundant
            tags are those previously registered tags that can now be formed
            using the subtags defined in the registry.  The grandfathered
            entries include those that can never be legal because they are
            'irregular' (that is, they do not match the 'langtag' production
            in Figure 1), are limited by rule (subtags such as 'nyn' and
            'min' look like the extlang production, but cannot be registered
            as extended language subtags), or their subtags are
            inappropriate for registration.  All of the grandfathered tags
            are listed in either the 'regular' or the 'irregular'
            productions in the ABNF.  Under [RFC4646] it was possible for
            grandfathered tags to become redundant.  However, all of the
            tags for which this was possible became redundant before this
            document was produced.  So the set of redundant and
            grandfathered tags is now permanent and immutable: new entries
            of either type MUST NOT be added and existing entries MUST NOT
            be removed.  The decision-making process about which tags were
            initially grandfathered and which were made redundant is
            described in [RFC4645].

            Many of the grandfathered tags are deprecated -- indeed, they
            were deprecated even before [RFC4646].  For example, the tag
            "art-lojban" was deprecated in favor of the primary language
            subtag 'jbo'.  These tags could have been made 'redundant' by
            registering some of their subtags as 'variants'.  The 'variant-
            like' subtags in the grandfathered registrations SHALL NOT be
            registered in the future, even with a similar or identical
            meaning.

    3.5.  Registration Procedure for Subtags

       The procedure given here MUST be used by anyone who wants to use a
       subtag not currently in the IANA Language Subtag Registry or who
       wishes to add, modify, update, or remove information in existing
       records as permitted by this document.

       Only subtags of type 'language' and 'variant' will be considered for
       independent registration of new subtags.  Subtags needed for

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       stability and subtags necessary to keep the registry synchronized
       with ISO 639, ISO 15924, ISO 3166, and UN M.49 within the limits
       defined by this document also use this process, as described in
       Section 3.3 and subject to stability provisions as described in
       Section 3.4.

       Registration requests are accepted relating to information in the
       'Comments', 'Deprecated', 'Description', 'Prefix', 'Preferred-Value',
       'Macrolanguage', or 'Suppress-Script' fields in a subtag's record as
       described in Section 3.4.  Changes to all other fields in the IANA
       registry are NOT permitted.

       Registering a new subtag or requesting modifications to an existing
       tag or subtag starts with the requester filling out the registration
       form reproduced below.  Note that each response is not limited in
       size so that the request can adequately describe the registration.
       The fields in the "Record Requested" section need to follow the
       requirements in Section 3.1 before the record will be approved.

       LANGUAGE SUBTAG REGISTRATION FORM
       1. Name of requester:
       2. E-mail address of requester:
       3. Record Requested:

          Type:
          Subtag:
          Description:
          Prefix:
          Preferred-Value:
          Deprecated:
          Suppress-Script:
          Macrolanguage:
          Comments:

       4. Intended meaning of the subtag:
       5. Reference to published description
          of the language (book or article):
       6. Any other relevant information:

                  Figure 5: The Language Subtag Registration Form

       Examples of completed registration forms can be found in Appendix B.
       A complete list of approved registration forms is online through
       http://www.iana.org; readers should note that the Language Tag
       Registry is now obsolete and should instead look for the Language
       Subtag Registry.

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       The subtag registration form MUST be sent to
       <ietf-languages@iana.org>.  Registration requests receive a two-week
       review period before being approved and submitted to IANA for
       inclusion in the registry.  If modifications are made to the request
       during the course of the registration process (such as corrections to
       meet the requirements in Section 3.1 or to make the 'Description'
       fields unique for the given record type), the modified form MUST also
       be sent to <ietf-languages@iana.org> at least one week prior to
       submission to IANA.

       The ietf-languages list is an open list and can be joined by sending
       a request to <ietf-languages-request@iana.org>.  The list can be
       hosted by IANA or any third party at the request of IESG.

       Before forwarding any registration to IANA, the Language Subtag
       Reviewer MUST ensure that all requirements in this document are met.
       This includes ensuring that values in the 'Subtag' field match case
       according to the description in Section 3.1.4 and that 'Description'
       fields are unique for the given record type as described in
       Section 3.1.5.  The Reviewer MUST also ensure that an appropriate
       File-Date record is included in the request, to assist IANA when
       updating the registry (see Section 5.1).

       Some fields in both the registration form as well as the registry
       record itself permit the use of non-ASCII characters.  Registration
       requests SHOULD use the UTF-8 encoding for consistency and clarity.
       However, since some mail clients do not support this encoding, other
       encodings MAY be used for the registration request.  The Language
       Subtag Reviewer is responsible for ensuring that the proper Unicode
       characters appear in both the archived request form and the registry
       record.  In the case of a transcription or encoding error by IANA,
       the Language Subtag Reviewer will request that the registry be
       repaired, providing any necessary information to assist IANA.

       Extended language subtags (type 'extlang'), by definition, are always
       encompassed by another language.  All records of type 'extlang' MUST,
       therefore, contain a 'Prefix' field at the time of registration.
       This 'Prefix' field can never be altered or removed, and requests to
       do so MUST be rejected.

       Variant subtags are usually registered for use with a particular
       range of language tags, and variant subtags based on the terminology
       of the language to which they are apply are encouraged.  For example,
       the subtag 'rozaj' (Resian) is intended for use with language tags
       that start with the primary language subtag "sl" (Slovenian), since
       Resian is a dialect of Slovenian.  Thus, the subtag 'rozaj' would be
       appropriate in tags such as "sl-Latn-rozaj" or "sl-IT-rozaj".  This
       information is stored in the 'Prefix' field in the registry.  Variant

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    RFC 5646                     Language Tags                September 2009

       registration requests SHOULD include at least one 'Prefix' field in
       the registration form.

       Requests to assign an additional record of a given type with an
       existing subtag value MUST be rejected.  For example, the variant
       subtag 'rozaj' already exists in the registry, so adding a second
       record of type 'variant' with the subtag 'rozaj' is prohibited.

       The 'Prefix' field for a given registered variant subtag exists in
       the IANA registry as a guide to usage.  Additional 'Prefix' fields
       MAY be added by filing an additional registration form.  In that
       form, the "Any other relevant information:" field MUST indicate that
       it is the addition of a prefix.

       Requests to add a 'Prefix' field to a variant subtag that imply a
       different semantic meaning SHOULD be rejected.  For example, a
       request to add the prefix "de" to the subtag '1994' so that the tag
       "de-1994" represented some German dialect or orthographic form would
       be rejected.  The '1994' subtag represents a particular Slovenian
       orthography, and the additional registration would change or blur the
       semantic meaning assigned to the subtag.  A separate subtag SHOULD be
       proposed instead.

       Requests to add a 'Prefix' to a variant subtag that has no current
       'Prefix' field MUST be rejected.  Variants are registered with no
       prefix because they are potentially useful with many or even all
       languages.  Adding one or more 'Prefix' fields would be potentially
       harmful to the use of the variant, since it dramatically reduces the
       scope of the subtag (which is not allowed under the stability rules
       (Section 3.4) as opposed to broadening the scope of the subtag, which
       is what the addition of a 'Prefix' normally does.  An example of such
       a "no-prefix" variant is the subtag 'fonipa', which represents the
       International Phonetic Alphabet, a scheme that can be used to
       transcribe many languages.

       The 'Description' fields provided in the request MUST contain at
       least one description written or transcribed into the Latin script;
       the request MAY also include additional 'Description' fields in any
       script or language.  The 'Description' field is used for
       identification purposes and doesn't necessarily represent the actual
       native name of the language or variation.  It also doesn't have to be
       in any particular language, but SHOULD be both suitable and
       sufficient to identify the item in the record.  The Language Subtag
       Reviewer will check and edit any proposed 'Description' fields so as
       to ensure uniqueness and prevent collisions with 'Description' fields
       in other records of the same type.  If this occurs in an independent
       registration request, the Language Subtag Reviewer MUST resubmit the
       record to <ietf-languages@iana.org>, treating it as a modification of

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       a request due to discussion, as described in Section 3.5, unless the
       request's sole purpose is to introduce a duplicate 'Description'
       field, in which case the request SHALL be rejected.

       The 'Description' field is not guaranteed to be stable.  Corrections
       or clarifications of intent are examples of possible changes.
       Attempts to provide translations or transcriptions of entries in the
       registry (which, by definition, provide no new information) are
       unlikely to be approved.

       Soon after the two-week review period has passed, the Language Subtag
       Reviewer MUST take one of the following actions:

       o  Explicitly accept the request and forward the form containing the
          record to be inserted or modified to <iana@iana.org> according to
          the procedure described in Section 3.3.

       o  Explicitly reject the request because of significant objections
          raised on the list or due to problems with constraints in this
          document (which MUST be explicitly cited).

       o  Extend the review period by granting an additional two-week
          increment to permit further discussion.  After each two-week
          increment, the Language Subtag Reviewer MUST indicate on the list
          whether the registration has been accepted, rejected, or extended.

       Note that the Language Subtag Reviewer MAY raise objections on the
       list if he or she so desires.  The important thing is that the
       objection MUST be made publicly.

       Sometimes the request needs to be modified as a result of discussion
       during the review period or due to requirements in this document.
       The applicant, Language Subtag Reviewer, or others MAY submit a
       modified version of the completed registration form, which will be
       considered in lieu of the original request with the explicit approval
       of the applicant.  Such changes do not restart the two-week
       discussion period, although an application containing the final
       record submitted to IANA MUST appear on the list at least one week
       prior to the Language Subtag Reviewer forwarding the record to IANA.
       The applicant MAY modify a rejected application with more appropriate
       or additional information and submit it again; this starts a new two-
       week comment period.

       Registrations initiated due to the provisions of Section 3.3 or
       Section 3.4 SHALL NOT be rejected altogether (since they have to
       ultimately appear in the registry) and SHOULD be completed as quickly
       as possible.  The review process allows list members to comment on
       the specific information in the form and the record it contains and

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    RFC 5646                     Language Tags                September 2009

       thus help ensure that it is correct and consistent.  The Language
       Subtag Reviewer MAY reject a specific version of the form, but MUST
       propose a suitable replacement, extending the review period as
       described above, until the form is in a format worthy of the
       reviewer's approval and meets with rough consensus of the list.

       Decisions made by the Language Subtag Reviewer MAY be appealed to the
       IESG [RFC2028] under the same rules as other IETF decisions
       [RFC2026].  This includes a decision to extend the review period or
       the failure to announce a decision in a clear and timely manner.

       The approved records appear in the Language Subtag Registry.  The
       approved registration forms are available online from
       http://www.iana.org.

       Updates or changes to existing records follow the same procedure as
       new registrations.  The Language Subtag Reviewer decides whether
       there is consensus to update the registration following the two-week
       review period; normally, objections by the original registrant will
       carry extra weight in forming such a consensus.

       Registrations are permanent and stable.  Once registered, subtags
       will not be removed from the registry and will remain a valid way in
       which to specify a specific language or variant.

       Note: The purpose of the "Reference to published description" section
       in the registration form is to aid in verifying whether a language is
       registered or to which language or language variation a particular
       subtag refers.  In most cases, reference to an authoritative grammar
       or dictionary of that language will be useful; in cases where no such
       work exists, other well-known works describing that language or in
       that language MAY be appropriate.  The Language Subtag Reviewer
       decides what constitutes "good enough" reference material.  This
       requirement is not intended to exclude particular languages or
       dialects due to the size of the speaker population or lack of a
       standardized orthography.  Minority languages will be considered
       equally on their own merits.

    3.6.  Possibilities for Registration

       Possibilities for registration of subtags or information about
       subtags include:

       o  Primary language subtags for languages not listed in ISO 639 that
          are not variants of any listed or registered language MAY be
          registered.  At the time this document was created, there were no
          examples of this form of subtag.  Before attempting to register a
          language subtag, there MUST be an attempt to register the language

    Phillips & Davis         Best Current Practice                 [Page 46]

    RFC 5646                     Language Tags                September 2009

          with ISO 639.  Subtags MUST NOT be registered for languages
          defined by codes that exist in ISO 639-1, ISO 639-2, or ISO 639-3;
          that are under consideration by the ISO 639 registration
          authorities; or that have never been attempted for registration
          with those authorities.  If ISO 639 has previously rejected a
          language for registration, it is reasonable to assume that there
          must be additional, very compelling evidence of need before it
          will be registered as a primary language subtag in the IANA
          registry (to the extent that it is very unlikely that any subtags
          will be registered of this type).

       o  Dialect or other divisions or variations within a language, its
          orthography, writing system, regional or historical usage,
          transliteration or other transformation, or distinguishing
          variation MAY be registered as variant subtags.  An example is the
          'rozaj' subtag (the Resian dialect of Slovenian).

       o  The addition or maintenance of fields (generally of an
          informational nature) in tag or subtag records as described in
          Section 3.1 is allowed.  Such changes are subject to the stability
          provisions in Section 3.4.  This includes 'Description',
          'Comments', 'Deprecated', and 'Preferred-Value' fields for
          obsolete or withdrawn codes, or the addition of 'Suppress-Script'
          or 'Macrolanguage' fields to primary language subtags, as well as
          other changes permitted by this document, such as the addition of
          an appropriate 'Prefix' field to a variant subtag.

       o  The addition of records and related field value changes necessary
          to reflect assignments made by ISO 639, ISO 15924, ISO 3166-1, and
          UN M.49 as described in Section 3.4 is allowed.

       Subtags proposed for registration that would cause all or part of a
       grandfathered tag to become redundant but whose meaning conflicts
       with or alters the meaning of the grandfathered tag MUST be rejected.

       This document leaves the decision on what subtags or changes to
       subtags are appropriate (or not) to the registration process
       described in Section 3.5.

       Note: Four-character primary language subtags are reserved to allow
       for the possibility of alpha4 codes in some future addition to the
       ISO 639 family of standards.

       ISO 639 defines a registration authority for additions to and changes
       in the list of languages in ISO 639.  This agency is:

    Phillips & Davis         Best Current Practice                 [Page 47]

    RFC 5646                     Language Tags                September 2009

       International Information Centre for Terminology (Infoterm)
       Aichholzgasse 6/12, AT-1120
       Wien, Austria
       Phone: +43 1 26 75 35 Ext. 312 Fax: +43 1 216 32 72

       ISO 639-2 defines a registration authority for additions to and
       changes in the list of languages in ISO 639-2.  This agency is:

       Library of Congress
       Network Development and MARC Standards Office
       Washington, DC 20540, USA
       Phone: +1 202 707 6237 Fax: +1 202 707 0115
       URL: http://www.loc.gov/standards/iso639-2

       ISO 639-3 defines a registration authority for additions to and
       changes in the list of languages in ISO 639-3.  This agency is:

       SIL International
       ISO 639-3 Registrar
       7500 W. Camp Wisdom Rd.
       Dallas, TX 75236, USA
       Phone: +1 972 708 7400, ext. 2293
       Fax: +1 972 708 7546
       Email: iso639-3@sil.org
       URL: http://www.sil.org/iso639-3

       ISO 639-5 defines a registration authority for additions to and
       changes in the list of languages in ISO 639-5.  This agency is the
       same as for ISO 639-2 and is:

       Library of Congress
       Network Development and MARC Standards Office
       Washington, DC 20540, USA
       Phone: +1 202 707 6237
       Fax: +1 202 707 0115
       URL: http://www.loc.gov/standards/iso639-5

       The maintenance agency for ISO 3166-1 (country codes) is:

       ISO 3166 Maintenance Agency
       c/o International Organization for Standardization
       Case postale 56
       CH-1211 Geneva 20, Switzerland
       Phone: +41 22 749 72 33 Fax: +41 22 749 73 49
       URL: http://www.iso.org/iso/en/prods-services/iso3166ma/index.html

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    RFC 5646                     Language Tags                September 2009

       The registration authority for ISO 15924 (script codes) is:

       Unicode Consortium
       Box 391476
       Mountain View, CA 94039-1476, USA
       URL: http://www.unicode.org/iso15924

       The Statistics Division of the United Nations Secretariat maintains
       the Standard Country or Area Codes for Statistical Use and can be
       reached at:

       Statistical Services Branch
       Statistics Division
       United Nations, Room DC2-1620
       New York, NY 10017, USA
       Fax: +1-212-963-0623
       Email: statistics@un.org
       URL: http://unstats.un.org/unsd/methods/m49/m49alpha.htm

    3.7.  Extensions and the Extensions Registry

       Extension subtags are those introduced by single-character subtags
       ("singletons") other than 'x'.  They are reserved for the generation
       of identifiers that contain a language component and are compatible
       with applications that understand language tags.

       The structure and form of extensions are defined by this document so
       that implementations can be created that are forward compatible with
       applications that might be created using singletons in the future.
       In addition, defining a mechanism for maintaining singletons will
       lend stability to this document by reducing the likely need for
       future revisions or updates.

       Single-character subtags are assigned by IANA using the "IETF Review"
       policy defined by [RFC5226].  This policy requires the development of
       an RFC, which SHALL define the name, purpose, processes, and
       procedures for maintaining the subtags.  The maintaining or
       registering authority, including name, contact email, discussion list
       email, and URL location of the registry, MUST be indicated clearly in
       the RFC.  The RFC MUST specify or include each of the following:

       o  The specification MUST reference the specific version or revision
          of this document that governs its creation and MUST reference this
          section of this document.

       o  The specification and all subtags defined by the specification
          MUST follow the ABNF and other rules for the formation of tags and
          subtags as defined in this document.  In particular, it MUST

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          specify that case is not significant and that subtags MUST NOT
          exceed eight characters in length.

       o  The specification MUST specify a canonical representation.

       o  The specification of valid subtags MUST be available over the
          Internet and at no cost.

       o  The specification MUST be in the public domain or available via a
          royalty-free license acceptable to the IETF and specified in the
          RFC.

       o  The specification MUST be versioned, and each version of the
          specification MUST be numbered, dated, and stable.

       o  The specification MUST be stable.  That is, extension subtags,
          once defined by a specification, MUST NOT be retracted or change
          in meaning in any substantial way.

       o  The specification MUST include, in a separate section, the
          registration form reproduced in this section (below) to be used in
          registering the extension upon publication as an RFC.

       o  IANA MUST be informed of changes to the contact information and
          URL for the specification.

       IANA will maintain a registry of allocated single-character
       (singleton) subtags.  This registry MUST use the record-jar format
       described by the ABNF in Section 3.1.1.  Upon publication of an
       extension as an RFC, the maintaining authority defined in the RFC
       MUST forward this registration form to <iesg@ietf.org>, who MUST
       forward the request to <iana@iana.org>.  The maintaining authority of
       the extension MUST maintain the accuracy of the record by sending an
       updated full copy of the record to <iana@iana.org> with the subject
       line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes.  Only
       the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY
       be modified in these updates.

       Failure to maintain this record, maintain the corresponding registry,
       or meet other conditions imposed by this section of this document MAY
       be appealed to the IESG [RFC2028] under the same rules as other IETF
       decisions (see [RFC2026]) and MAY result in the authority to maintain
       the extension being withdrawn or reassigned by the IESG.

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       %%
       Identifier:
       Description:
       Comments:
       Added:
       RFC:
       Authority:
       Contact_Email:
       Mailing_List:
       URL:
       %%

        Figure 6: Format of Records in the Language Tag Extensions Registry

       'Identifier' contains the single-character subtag (singleton)
       assigned to the extension.  The Internet-Draft submitted to define
       the extension SHOULD specify which letter or digit to use, although
       the IESG MAY change the assignment when approving the RFC.

       'Description' contains the name and description of the extension.

       'Comments' is an OPTIONAL field and MAY contain a broader description
       of the extension.

       'Added' contains the date the extension's RFC was published in the
       "full-date" format specified in [RFC3339].  For example: 2004-06-28
       represents June 28, 2004, in the Gregorian calendar.

       'RFC' contains the RFC number assigned to the extension.

       'Authority' contains the name of the maintaining authority for the
       extension.

       'Contact_Email' contains the email address used to contact the
       maintaining authority.

       'Mailing_List' contains the URL or subscription email address of the
       mailing list used by the maintaining authority.

       'URL' contains the URL of the registry for this extension.

       The determination of whether an Internet-Draft meets the above
       conditions and the decision to grant or withhold such authority rests
       solely with the IESG and is subject to the normal review and appeals
       process associated with the RFC process.

       Extension authors are strongly cautioned that many (including most
       well-formed) processors will be unaware of any special relationships

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       or meaning inherent in the order of extension subtags.  Extension
       authors SHOULD avoid subtag relationships or canonicalization
       mechanisms that interfere with matching or with length restrictions
       that sometimes exist in common protocols where the extension is used.
       In particular, applications MAY truncate the subtags in doing
       matching or in fitting into limited lengths, so it is RECOMMENDED
       that the most significant information be in the most significant
       (left-most) subtags and that the specification gracefully handle
       truncated subtags.

       When a language tag is to be used in a specific, known protocol, it
       is RECOMMENDED that the language tag not contain extensions not
       supported by that protocol.  In addition, note that some protocols
       MAY impose upper limits on the length of the strings used to store or
       transport the language tag.

    3.8.  Update of the Language Subtag Registry

       After the adoption of this document, the IANA Language Subtag
       Registry needed an update so that it would contain the complete set
       of subtags valid in a language tag.  [RFC5645] describes the process
       used to create this update.

       Registrations that are in process under the rules defined in
       [RFC4646] when this document is adopted MUST be completed under the
       rules contained in this document.

    3.9.  Applicability of the Subtag Registry

       The Language Subtag Registry is the source of data elements used to
       construct language tags, following the rules described in this
       document.  Language tags are designed for indicating linguistic
       attributes of various content, including not only text but also most
       media formats, such as video or audio.  They also form the basis for
       language and locale negotiation in various protocols and APIs.

       The registry is therefore applicable to many applications that need
       some form of language identification, with these limitations:

       o  It is not designed to be the sole data source in the creation of a
          language-selection user interface.  For example, the registry does
          not contain translations for subtag descriptions or for tags
          composed from the subtags.  Sources for localized data based on
          the registry are generally available, notably [CLDR].  Nor does
          the registry indicate which subtag combinations are particularly
          useful or relevant.

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       o  It does not provide information indicating relationships between
          different languages, such as might be used in a user interface to
          select language tags hierarchically, regionally, or on some other
          organizational model.

       o  It does not supply information about potential overlap between
          different language tags, as the notion of what constitutes a
          language is not precise: several different language tags might be
          reasonable choices for the same given piece of content.

       o  It does not contain information about appropriate fallback choices
          when performing language negotiation.  A good fallback language
          might be linguistically unrelated to the specified language.  The
          fact that one language is often used as a fallback language for
          another is usually a result of outside factors, such as geography,
          history, or culture -- factors that might not apply in all cases.
          For example, most people who use Breton (a Celtic language used in
          the Northwest of France) would probably prefer to be served French
          (a Romance language) if Breton isn't available.

    4.  Formation and Processing of Language Tags

       This section addresses how to use the information in the registry
       with the tag syntax to choose, form, and process language tags.

    4.1.  Choice of Language Tag

       The guiding principle in forming language tags is to "tag content
       wisely."  Sometimes there is a choice between several possible tags
       for the same content.  The choice of which tag to use depends on the
       content and application in question, and some amount of judgment
       might be necessary when selecting a tag.

       Interoperability is best served when the same language tag is used
       consistently to represent the same language.  If an application has
       requirements that make the rules here inapplicable, then that
       application risks damaging interoperability.  It is strongly
       RECOMMENDED that users not define their own rules for language tag
       choice.

       Standards, protocols, and applications that reference this document
       normatively but apply different rules to the ones given in this
       section MUST specify how language tag selection varies from the
       guidelines given here.

       To ensure consistent backward compatibility, this document contains
       several provisions to account for potential instability in the
       standards used to define the subtags that make up language tags.

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       These provisions mean that no valid language tag can become invalid,
       nor will a language tag have a narrower scope in the future (it may
       have a broader scope).  The most appropriate language tag for a given
       application or content item might evolve over time, but once applied,
       the tag itself cannot become invalid or have its meaning wholly
       change.

       A subtag SHOULD only be used when it adds useful distinguishing
       information to the tag.  Extraneous subtags interfere with the
       meaning, understanding, and processing of language tags.  In
       particular, users and implementations SHOULD follow the 'Prefix' and
       'Suppress-Script' fields in the registry (defined in Section 3.1):
       these fields provide guidance on when specific additional subtags
       SHOULD be used or avoided in a language tag.

       The choice of subtags used to form a language tag SHOULD follow these
       guidelines:

       1.  Use as precise a tag as possible, but no more specific than is
           justified.  Avoid using subtags that are not important for
           distinguishing content in an application.

           *  For example, 'de' might suffice for tagging an email written
              in German, while "de-CH-1996" is probably unnecessarily
              precise for such a task.

           *  Note that some subtag sequences might not represent the
              language a casual user might expect.  For example, the Swiss
              German (Schweizerdeutsch) language is represented by "gsw-CH"
              and not by "de-CH".  This latter tag represents German ('de')
              as used in Switzerland ('CH'), also known as Swiss High German
              (Schweizer Hochdeutsch).  Both are real languages, and
              distinguishing between them could be important to an
              application.

       2.  The script subtag SHOULD NOT be used to form language tags unless
           the script adds some distinguishing information to the tag.
           Script subtags were first formally defined in [RFC4646].  Their
           use can affect matching and subtag identification for
           implementations of [RFC1766] or [RFC3066] (which are obsoleted by
           this document), as these subtags appear between the primary
           language and region subtags.  Some applications can benefit from
           the use of script subtags in language tags, as long as the use is
           consistent for a given context.  Script subtags are never
           appropriate for unwritten content (such as audio recordings).
           The field 'Suppress-Script' in the primary or extended language
           record in the registry indicates script subtags that do not add
           distinguishing information for most applications; this field

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           defines when users SHOULD NOT include a script subtag with a
           particular primary language subtag.

           For example, if an implementation selects content using Basic
           Filtering [RFC4647] (originally described in Section 14.4 of
           [RFC2616]) and the user requested the language range "en-US",
           content labeled "en-Latn-US" will not match the request and thus
           not be selected.  Therefore, it is important to know when script
           subtags will customarily be used and when they ought not be used.

           For example:

           *  The subtag 'Latn' should not be used with the primary language
              'en' because nearly all English documents are written in the
              Latin script and it adds no distinguishing information.
              However, if a document were written in English mixing Latin
              script with another script such as Braille ('Brai'), then it
              might be appropriate to choose to indicate both scripts to aid
              in content selection, such as the application of a style
              sheet.

           *  When labeling content that is unwritten (such as a recording
              of human speech), the script subtag should not be used, even
              if the language is customarily written in several scripts.
              Thus, the subtitles to a movie might use the tag "uz-Arab"
              (Uzbek, Arabic script), but the audio track for the same
              language would be tagged simply "uz".  (The tag "uz-Zxxx"
              could also be used where content is not written, as the subtag
              'Zxxx' represents the "Code for unwritten documents".)

       3.  If a tag or subtag has a 'Preferred-Value' field in its registry
           entry, then the value of that field SHOULD be used to form the
           language tag in preference to the tag or subtag in which the
           preferred value appears.

           *  For example, use 'jbo' for Lojban in preference to the
              grandfathered tag "art-lojban".

       4.  Use subtags or sequences of subtags for individual languages in
           preference to subtags for language collections.  A "language
           collection" is a group of languages that are descended from a
           common ancestor, are spoken in the same geographical area, or are
           otherwise related.  Certain language collections are assigned
           codes by [ISO639-5] (and some of these [ISO639-5] codes are also
           defined as collections in [ISO639-2]).  These codes are included
           as primary language subtags in the registry.  Subtags for a
           language collection in the registry have a 'Scope' field with a
           value of 'collection'.  A subtag for a language collection is

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           always preferred to less specific alternatives such as 'mul' and
           'und' (see below), and a subtag representing a language
           collection MAY be used when more specific language information is
           not available.  However, most users and implementations do not
           know there is a relationship between the collection and its
           individual languages.  In addition, the relationship between the
           individual languages in the collection is not well defined; in
           particular, the languages are usually not mutually intelligible.
           Since the subtags are different, a request for the collection
           will typically only produce items tagged with the collection's
           subtag, not items tagged with subtags for the individual
           languages contained in the collection.

           *  For example, collections are interpreted inclusively, so the
              subtag 'gem' (Germanic languages) could, but SHOULD NOT, be
              used with content that would be better tagged with "en"
              (English), "de" (German), or "gsw" (Swiss German, Alemannic).
              While 'gem' collects all of these (and other) languages, most
              implementations will not match 'gem' to the individual
              languages; thus, using the subtag will not produce the desired
              result.

       5.  [ISO639-2] has defined several codes included in the subtag
           registry that require additional care when choosing language
           tags.  In most of these cases, where omitting the language tag is
           permitted, such omission is preferable to using these codes.
           Language tags SHOULD NOT incorporate these subtags as a prefix,
           unless the additional information conveys some value to the
           application.

           *  The 'mul' (Multiple) primary language subtag identifies
              content in multiple languages.  This subtag SHOULD NOT be used
              when a list of languages or individual tags for each content
              element can be used instead.  For example, the 'Content-
              Language' header [RFC3282] allows a list of languages to be
              used, not just a single language tag.

           *  The 'und' (Undetermined) primary language subtag identifies
              linguistic content whose language is not determined.  This
              subtag SHOULD NOT be used unless a language tag is required
              and language information is not available or cannot be
              determined.  Omitting the language tag (where permitted) is
              preferred.  The 'und' subtag might be useful for protocols
              that require a language tag to be provided or where a primary
              language subtag is required (such as in "und-Latn").  The
              'und' subtag MAY also be useful when matching language tags in
              certain situations.

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           *  The 'zxx' (Non-Linguistic, Not Applicable) primary language
              subtag identifies content for which a language classification
              is inappropriate or does not apply.  Some examples might
              include instrumental or electronic music; sound recordings
              consisting of nonverbal sounds; audiovisual materials with no
              narration, dialog, printed titles, or subtitles; machine-
              readable data files consisting of machine languages or
              character codes; or programming source code.

           *  The 'mis' (Uncoded) primary language subtag identifies content
              whose language is known but that does not currently have a
              corresponding subtag.  This subtag SHOULD NOT be used.
              Because the addition of other codes in the future can render
              its application invalid, it is inherently unstable and hence
              incompatible with the stability goals of BCP 47.  It is always
              preferable to use other subtags: either 'und' or (with prior
              agreement) private use subtags.

       6.  Use variant subtags sparingly and in the correct order.  Most
           variant subtags have one or more 'Prefix' fields in the registry
           that express the list of subtags with which they are appropriate.
           Variants SHOULD only be used with subtags that appear in one of
           these 'Prefix' fields.  If a variant lists a second variant in
           one of its 'Prefix' fields, the first variant SHOULD appear
           directly after the second variant in any language tag where both
           occur.  General purpose variants (those with no 'Prefix' fields
           at all) SHOULD appear after any other variant subtags.  Order any
           remaining variants by placing the most significant subtag first.
           If none of the subtags is more significant or no relationship can
           be determined, alphabetize the subtags.  Because variants are
           very specialized, using many of them together generally makes the
           tag so narrow as to override the additional precision gained.
           Putting the subtags into another order interferes with
           interoperability, as well as the overall interpretation of the
           tag.

           For example:

           *  The tag "en-scotland-fonipa" (English, Scottish dialect, IPA
              phonetic transcription) is correctly ordered because
              'scotland' has a 'Prefix' of "en", while 'fonipa' has no
              'Prefix' field.

           *  The tag "sl-IT-rozaj-biske-1994" is correctly ordered: 'rozaj'
              lists "sl" as its sole 'Prefix'; 'biske' lists "sl-rozaj" as
              its sole 'Prefix'.  The subtag '1994' has several prefixes,

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              including "sl-rozaj".  However, it follows both 'rozaj' and
              'biske' because one of its 'Prefix' fields is "sl-rozaj-
              biske".

       7.  The grandfathered tag "i-default" (Default Language) was
           originally registered according to [RFC1766] to meet the needs of
           [RFC2277].  It is not used to indicate a specific language, but
           rather to identify the condition or content used where the
           language preferences of the user cannot be established.  It
           SHOULD NOT be used except as a means of labeling the default
           content for applications or protocols that require default
           language content to be labeled with that specific tag.  It MAY
           also be used by an application or protocol to identify when the
           default language content is being returned.

    4.1.1.  Tagging Encompassed Languages

       Some primary language records in the registry have a 'Macrolanguage'
       field (Section 3.1.10) that contains a mapping from each "encompassed
       language" to its macrolanguage.  The 'Macrolanguage' mapping doesn't
       define what the relationship between the encompassed language and its
       macrolanguage is, nor does it define how languages encompassed by the
       same macrolanguage are related to each other.  Two different
       languages encompassed by the same macrolanguage may differ from one
       another more than, say, French and Spanish do.

       A few specific macrolanguages, such as Chinese ('zh') and Arabic
       ('ar'), are handled differently.  See Section 4.1.2.

       The more specific encompassed language subtag SHOULD be used to form
       the language tag, although either the macrolanguage's primary
       language subtag or the encompassed language's subtag MAY be used.
       This means, for example, tagging Plains Cree with 'crk' rather than
       'cr' (Cree), and so forth.

       Each macrolanguage subtag's scope, by definition, includes all of its
       encompassed languages.  Since the relationship between encompassed
       languages varies, users cannot assume that the macrolanguage subtag
       means any particular encompassed language, nor that any given pair of
       encompassed languages are mutually intelligible or otherwise
       interchangeable.

       Applications MAY use macrolanguage information to improve matching or
       language negotiation.  For example, the information that 'sr'
       (Serbian) and 'hr' (Croatian) share a macrolanguage expresses a
       closer relation between those languages than between, say, 'sr'
       (Serbian) and 'ma' (Macedonian).  However, this relationship is not
       guaranteed nor is it exclusive.  For example, Romanian ('ro') and

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       Moldavian ('mo') do not share a macrolanguage, but are far more
       closely related to each other than Cantonese ('yue') and Wu ('wuu'),
       which do share a macrolanguage.

    4.1.2.  Using Extended Language Subtags

       To accommodate language tag forms used prior to the adoption of this
       document, language tags provide a special compatibility mechanism:
       the extended language subtag.  Selected languages have been provided
       with both primary and extended language subtags.  These include
       macrolanguages, such as Malay ('ms') and Uzbek ('uz'), that have a
       specific dominant variety that is generally synonymous with the
       macrolanguage.  Other languages, such as the Chinese ('zh') and
       Arabic ('ar') macrolanguages and the various sign languages ('sgn'),
       have traditionally used their primary language subtag, possibly
       coupled with various region subtags or as part of a registered
       grandfathered tag, to indicate the language.

       With the adoption of this document, specific ISO 639-3 subtags became
       available to identify the languages contained within these diverse
       language families or groupings.  This presents a choice of language
       tags where previously none existed:

       o  Each encompassed language's subtag SHOULD be used as the primary
          language subtag.  For example, a document in Mandarin Chinese
          would be tagged "cmn" (the subtag for Mandarin Chinese) in
          preference to "zh" (Chinese).

       o  If compatibility is desired or needed, the encompassed subtag MAY
          be used as an extended language subtag.  For example, a document
          in Mandarin Chinese could be tagged "zh-cmn" instead of either
          "cmn" or "zh".

       o  The macrolanguage or prefixing subtag MAY still be used to form
          the tag instead of the more specific encompassed language subtag.
          That is, tags such as "zh-HK" or "sgn-RU" are still valid.

       Chinese ('zh') provides a useful illustration of this.  In the past,
       various content has used tags beginning with the 'zh' subtag, with
       application-specific meaning being associated with region codes,
       private use sequences, or grandfathered registered values.  This is
       because historically only the macrolanguage subtag 'zh' was available
       for forming language tags.  However, the languages encompassed by the
       Chinese subtag 'zh' are, in the main, not mutually intelligible when
       spoken, and the written forms of these languages also show wide
       variation in form and usage.

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       To provide compatibility, Chinese languages encompassed by the 'zh'
       subtag are in the registry both as primary language subtags and as
       extended language subtags.  For example, the ISO 639-3 code for
       Cantonese is 'yue'.  Content in Cantonese might historically have
       used a tag such as "zh-HK" (since Cantonese is commonly spoken in
       Hong Kong), although that tag actually means any type of Chinese as
       used in Hong Kong.  With the availability of ISO 639-3 codes in the
       registry, content in Cantonese can be directly tagged using the 'yue'
       subtag.  The content can use it as a primary language subtag, as in
       the tag "yue-HK" (Cantonese, Hong Kong).  Or it can use an extended
       language subtag with 'zh', as in the tag "zh-yue-Hant" (Chinese,
       Cantonese, Traditional script).

       As noted above, applications can choose to use the macrolanguage
       subtag to form the tag instead of using the more specific encompassed
       language subtag.  For example, an application with large quantities
       of data already using tags with the 'zh' (Chinese) subtag might
       continue to use this more general subtag even for new data, even
       though the content could be more precisely tagged with 'cmn'
       (Mandarin), 'yue' (Cantonese), 'wuu' (Wu), and so on.  Similarly, an
       application already using tags that start with the 'ar' (Arabic)
       subtag might continue to use this more general subtag even for new
       data, which could be more precisely tagged with 'arb' (Standard
       Arabic).

       In some cases, the encompassed languages had tags registered for them
       during the RFC 3066 era.  Those grandfathered tags not already
       deprecated or rendered redundant were deprecated in the registry upon
       adoption of this document.  As grandfathered values, they remain
       valid for use, and some content or applications might use them.  As
       with other grandfathered tags, since implementations might not be
       able to associate the grandfathered tags with the encompassed
       language subtag equivalents that are recommended by this document,
       implementations are encouraged to canonicalize tags for comparison
       purposes.  Some examples of this include the tags "zh-hakka" (Hakka)
       and "zh-guoyu" (Mandarin or Standard Chinese).

       Sign languages share a mode of communication rather than a linguistic
       heritage.  There are many sign languages that have developed
       independently, and the subtag 'sgn' indicates only the presence of a
       sign language.  A number of sign languages also had grandfathered
       tags registered for them during the RFC 3066 era.  For example, the
       grandfathered tag "sgn-US" was registered to represent 'American Sign
       Language' specifically, without reference to the United States.  This
       is still valid, but deprecated: a document in American Sign Language
       can be labeled either "ase" or "sgn-ase" (the 'ase' subtag is for the
       language called 'American Sign Language').

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    4.2.  Meaning of the Language Tag

       The meaning of a language tag is related to the meaning of the
       subtags that it contains.  Each subtag, in turn, implies a certain
       range of expectations one might have for related content, although it
       is not a guarantee.  For example, the use of a script subtag such as
       'Arab' (Arabic script) does not mean that the content contains only
       Arabic characters.  It does mean that the language involved is
       predominantly in the Arabic script.  Thus, a language tag and its
       subtags can encompass a very wide range of variation and yet remain
       appropriate in each particular instance.

       Validity of a tag is not the only factor determining its usefulness.
       While every valid tag has a meaning, it might not represent any real-
       world language usage.  This is unavoidable in a system in which
       subtags can be combined freely.  For example, tags such as
       "ar-Cyrl-CO" (Arabic, Cyrillic script, as used in Colombia) or "tlh-
       Kore-AQ-fonipa" (Klingon, Korean script, as used in Antarctica, IPA
       phonetic transcription) are both valid and unlikely to represent a
       useful combination of language attributes.

       The meaning of a given tag doesn't depend on the context in which it
       appears.  The relationship between a tag's meaning and the
       information objects to which that tag is applied, however, can vary.

       o  For a single information object, the associated language tags
          might be interpreted as the set of languages that is necessary for
          a complete comprehension of the complete object.  Example: Plain
          text documents.

       o  For an aggregation of information objects, the associated language
          tags could be taken as the set of languages used inside components
          of that aggregation.  Examples: Document stores and libraries.

       o  For information objects whose purpose is to provide alternatives,
          the associated language tags could be regarded as a hint that the
          content is provided in several languages and that one has to
          inspect each of the alternatives in order to find its language or
          languages.  In this case, the presence of multiple tags might not
          mean that one needs to be multilingual to get complete
          understanding of the document.  Example: MIME multipart/
          alternative [RFC2046].

       o  For markup languages, such as HTML and XML, language information
          can be added to each part of the document identified by the markup
          structure (including the whole document itself).  For example, one
          could write <span lang="fr">C'est la vie.</span> inside a German
          document; the German-speaking user could then access a French-

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          German dictionary to find out what the marked section meant.  If
          the user were listening to that document through a speech
          synthesis interface, this formation could be used to signal the
          synthesizer to appropriately apply French text-to-speech
          pronunciation rules to that span of text, instead of applying the
          inappropriate German rules.

       o  For markup languages and document formats that allow the audience
          to be identified, a language tag could indicate the audience(s)
          appropriate for that document.  For example, the same HTML
          document described in the preceding bullet might have an HTTP
          header "Content-Language: de" to indicate that the intended
          audience for the file is German (even though three words appear
          and are identified as being in French within it).

       o  For systems and APIs, language tags form the basis for most
          implementations of locale identifiers.  For example, see Unicode's
          CLDR (Common Locale Data Repository) (see UTS #35 [UTS35])
          project.

       Language tags are related when they contain a similar sequence of
       subtags.  For example, if a language tag B contains language tag A as
       a prefix, then B is typically "narrower" or "more specific" than A.
       Thus, "zh-Hant-TW" is more specific than "zh-Hant".

       This relationship is not guaranteed in all cases: specifically,
       languages that begin with the same sequence of subtags are NOT
       guaranteed to be mutually intelligible, although they might be.  For
       example, the tag "az" shares a prefix with both "az-Latn"
       (Azerbaijani written using the Latin script) and "az-Cyrl"
       (Azerbaijani written using the Cyrillic script).  A person fluent in
       one script might not be able to read the other, even though the
       linguistic content (e.g., what would be heard if both texts were read
       aloud) might be identical.  Content tagged as "az" most probably is
       written in just one script and thus might not be intelligible to a
       reader familiar with the other script.

       Similarly, not all subtags specify an actual distinction in language.
       For example, the tags "en-US" and "en-CA" mean, roughly, English with
       features generally thought to be characteristic of the United States
       and Canada, respectively.  They do not imply that a significant
       dialectical boundary exists between any arbitrarily selected point in
       the United States and any arbitrarily selected point in Canada.
       Neither does a particular region subtag imply that linguistic
       distinctions do not exist within that region.

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    4.3.  Lists of Languages

       In some applications, a single content item might best be associated
       with more than one language tag.  Examples of such a usage include:

       o  Content items that contain multiple, distinct varieties.  Often
          this is used to indicate an appropriate audience for a given
          content item when multiple choices might be appropriate.  Examples
          of this could include:

          *  Metadata about the appropriate audience for a movie title.  For
             example, a DVD might label its individual audio tracks 'de'
             (German), 'fr' (French), and 'es' (Spanish), but the overall
             title would list "de, fr, es" as its overall audience.

          *  A French/English, English/French dictionary tagged as both "en"
             and "fr" to specify that it applies equally to French and
             English.

          *  A side-by-side or interlinear translation of a document, as is
             commonly done with classical works in Latin or Greek.

       o  Content items that contain a single language but that require
          multiple levels of specificity.  For example, a library might wish
          to classify a particular work as both Norwegian ('no') and as
          Nynorsk ('nn') for audiences capable of appreciating the
          distinction or needing to select content more narrowly.

    4.4.  Length Considerations

       There is no defined upper limit on the size of language tags.  While
       historically most language tags have consisted of language and region
       subtags with a combined total length of up to six characters, larger
       tags have always been both possible and have actually appeared in
       use.

       Neither the language tag syntax nor other requirements in this
       document impose a fixed upper limit on the number of subtags in a
       language tag (and thus an upper bound on the size of a tag).  The
       language tag syntax suggests that, depending on the specific
       language, more subtags (and thus a longer tag) are sometimes
       necessary to completely identify the language for certain
       applications; thus, it is possible to envision long or complex subtag
       sequences.

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    4.4.1.  Working with Limited Buffer Sizes

       Some applications and protocols are forced to allocate fixed buffer
       sizes or otherwise limit the length of a language tag.  A conformant
       implementation or specification MAY refuse to support the storage of
       language tags that exceed a specified length.  Any such limitation
       SHOULD be clearly documented, and such documentation SHOULD include
       what happens to longer tags (for example, whether an error value is
       generated or the language tag is truncated).  A protocol that allows
       tags to be truncated at an arbitrary limit, without giving any
       indication of what that limit is, has the potential to cause harm by
       changing the meaning of tags in substantial ways.

       In practice, most language tags do not require more than a few
       subtags and will not approach reasonably sized buffer limitations;
       see Section 4.1.

       Some specifications or protocols have limits on tag length but do not
       have a fixed length limitation.  For example, [RFC2231] has no
       explicit length limitation: the length available for the language tag
       is constrained by the length of other header components (such as the
       charset's name) coupled with the 76-character limit in [RFC2047].
       Thus, the "limit" might be 50 or more characters, but it could
       potentially be quite small.

       The considerations for assigning a buffer limit are:

          Implementations SHOULD NOT truncate language tags unless the
          meaning of the tag is purposefully being changed, or unless the
          tag does not fit into a limited buffer size specified by a
          protocol for storage or transmission.

          Implementations SHOULD warn the user when a tag is truncated since
          truncation changes the semantic meaning of the tag.

          Implementations of protocols or specifications that are space
          constrained but do not have a fixed limit SHOULD use the longest
          possible tag in preference to truncation.

          Protocols or specifications that specify limited buffer sizes for
          language tags MUST allow for language tags of at least 35
          characters.  Note that [RFC4646] recommended a minimum field size
          of 42 characters because it included all three elements of the
          'extlang' production.  Two of these are now permanently reserved,
          so a registered primary language subtag of the maximum length of 8
          characters is now longer than the longest language-extlang
          combination.  Protocols or specifications that commonly use

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          extensions or private use subtags might wish to reserve or
          recommend a longer "minimum buffer" size.

       The following illustration shows how the 35-character recommendation
       was derived:

       language      =  8 ; longest allowed registered value
                          ;   longer than primary+extlang
                          ;   which requires 7 characters
       script        =  5 ; if not suppressed: see Section 4.1
       region        =  4 ; UN M.49 numeric region code
                          ;   ISO 3166-1 codes require 3
       variant1      =  9 ; needs 'language' as a prefix
       variant2      =  9 ; very rare, as it needs
                          ;   'language-variant1' as a prefix

       total         = 35 characters

                  Figure 7: Derivation of the Limit on Tag Length

    4.4.2.  Truncation of Language Tags

       Truncation of a language tag alters the meaning of the tag, and thus
       SHOULD be avoided.  However, truncation of language tags is sometimes
       necessary due to limited buffer sizes.  Such truncation MUST NOT
       permit a subtag to be chopped off in the middle or the formation of
       invalid tags (for example, one ending with the "-" character).

       This means that applications or protocols that truncate tags MUST do
       so by progressively removing subtags along with their preceding "-"
       from the right side of the language tag until the tag is short enough
       for the given buffer.  If the resulting tag ends with a single-
       character subtag, that subtag and its preceding "-" MUST also be
       removed.  For example:

       Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
       1. zh-Latn-CN-variant1-a-extend1-x-wadegile
       2. zh-Latn-CN-variant1-a-extend1
       3. zh-Latn-CN-variant1
       4. zh-Latn-CN
       5. zh-Latn
       6. zh

                        Figure 8: Example of Tag Truncation

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    4.5.  Canonicalization of Language Tags

       Since a particular language tag can be used by many processes,
       language tags SHOULD always be created or generated in canonical
       form.

       A language tag is in 'canonical form' when the tag is well-formed
       according to the rules in Sections 2.1 and 2.2 and it has been
       canonicalized by applying each of the following steps in order, using
       data from the IANA registry (see Section 3.1):

       1.  Extension sequences are ordered into case-insensitive ASCII order
           by singleton subtag.

           *  For example, the subtag sequence '-a-babble' comes before
              '-b-warble'.

       2.  Redundant or grandfathered tags are replaced by their 'Preferred-
           Value', if there is one.

           *  The field-body of the 'Preferred-Value' for grandfathered and
              redundant tags is an "extended language range" [RFC4647] and
              might consist of more than one subtag.

           *  'Preferred-Value' fields in the registry provide mappings from
              deprecated tags to modern equivalents.  Many of these were
              created before the adoption of this document (such as the
              mapping of "no-nyn" to "nn" or "i-klingon" to "tlh").  Others
              are the result of later registrations or additions to the
              registry as permitted or required by this document (for
              example, "zh-hakka" was deprecated in favor of the ISO 639-3
              code 'hak' when this document was adopted).

       3.  Subtags are replaced by their 'Preferred-Value', if there is one.
           For extlangs, the original primary language subtag is also
           replaced if there is a primary language subtag in the 'Preferred-
           Value'.

           *  The field-body of the 'Preferred-Value' for extlangs is an
              "extended language range" and typically maps to a primary
              language subtag.  For example, the subtag sequence "zh-hak"
              (Chinese, Hakka) is replaced with the subtag 'hak' (Hakka).

           *  Most of the non-extlang subtags are either Region subtags
              where the country name or designation has changed or clerical
              corrections to ISO 639-1.

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       The canonical form contains no 'extlang' subtags.  There is an
       alternate 'extlang form' that maintains or reinstates extlang
       subtags.  This form can be useful in environments where the presence
       of the 'Prefix' subtag is considered beneficial in matching or
       selection (see Section 4.1.2).

       A language tag is in 'extlang form' when the tag is well-formed
       according to the rules in Sections 2.1 and 2.2 and it has been
       processed by applying each of the following two steps in order, using
       data from the IANA registry:

       1.  The language tag is first transformed into canonical form, as
           described above.

       2.  If the language tag starts with a primary language subtag that is
           also an extlang subtag, then the language tag is prepended with
           the extlang's 'Prefix'.

           *  For example, "hak-CN" (Hakka, China) has the primary language
              subtag 'hak', which in turn has an 'extlang' record with a
              'Prefix' 'zh' (Chinese).  The extlang form is "zh-hak-CN"
              (Chinese, Hakka, China).

           *  Note that Step 2 (prepending a prefix) can restore a subtag
              that was removed by Step 1 (canonicalizing).

       Example: The language tag "en-a-aaa-b-ccc-bbb-x-xyz" is in canonical
       form, while "en-b-ccc-bbb-a-aaa-X-xyz" is well-formed and potentially
       valid (extensions 'a' and 'b' are not defined as of the publication
       of this document) but not in canonical form (the extensions are not
       in alphabetical order).

       Example: Although the tag "en-BU" (English as used in Burma)
       maintains its validity, the language tag "en-BU" is not in canonical
       form because the 'BU' subtag has a canonical mapping to 'MM'
       (Myanmar).

       Canonicalization of language tags does not imply anything about the
       use of upper- or lowercase letters when processing or comparing
       subtags (and as described in Section 2.1).  All comparisons MUST be
       performed in a case-insensitive manner.

       When performing canonicalization of language tags, processors MAY
       regularize the case of the subtags (that is, this process is
       OPTIONAL), following the case used in the registry (see
       Section 2.1.1).

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       If more than one variant appears within a tag, processors MAY reorder
       the variants to obtain better matching behavior or more consistent
       presentation.  Reordering of the variants SHOULD follow the
       recommendations for variant ordering in Section 4.1.

       If the field 'Deprecated' appears in a registry record without an
       accompanying 'Preferred-Value' field, then that tag or subtag is
       deprecated without a replacement.  These values are canonical when
       they appear in a language tag.  However, tags that include these
       values SHOULD NOT be selected by users or generated by
       implementations.

       An extension MUST define any relationships that exist between the
       various subtags in the extension and thus MAY define an alternate
       canonicalization scheme for the extension's subtags.  Extensions MAY
       define how the order of the extension's subtags is interpreted.  For
       example, an extension could define that its subtags are in canonical
       order when the subtags are placed into ASCII order: that is, "en-a-
       aaa-bbb-ccc" instead of "en-a-ccc-bbb-aaa".  Another extension might
       define that the order of the subtags influences their semantic
       meaning (so that "en-b-ccc-bbb-aaa" has a different value from "en-b-
       aaa-bbb-ccc").  However, extension specifications SHOULD be designed
       so that they are tolerant of the typical processes described in
       Section 3.7.

    4.6.  Considerations for Private Use Subtags

       Private use subtags, like all other subtags, MUST conform to the
       format and content constraints in the ABNF.  Private use subtags have
       no meaning outside the private agreement between the parties that
       intend to use or exchange language tags that employ them.  The same
       subtags MAY be used with a different meaning under a separate private
       agreement.  They SHOULD NOT be used where alternatives exist and
       SHOULD NOT be used in content or protocols intended for general use.

       Private use subtags are simply useless for information exchange
       without prior arrangement.  The value and semantic meaning of private
       use tags and of the subtags used within such a language tag are not
       defined by this document.

       Private use sequences introduced by the 'x' singleton are completely
       opaque to users or implementations outside of the private use
       agreement.  So, in addition to private use subtag sequences
       introduced by the singleton subtag 'x', the Language Subtag Registry
       provides private use language, script, and region subtags derived
       from the private use codes assigned by the underlying standards.
       These subtags are valid for use in forming language tags; they are
       RECOMMENDED over the 'x' singleton private use subtag sequences

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       because they convey more information via their linkage to the
       language tag's inherent structure.

       For example, the region subtags 'AA', 'ZZ', and those in the ranges
       'QM'-'QZ' and 'XA'-'XZ' (derived from the ISO 3166-1 private use
       codes) can be used to form a language tag.  A tag such as
       "zh-Hans-XQ" conveys a great deal of public, interchangeable
       information about the language material (that it is Chinese in the
       simplified Chinese script and is suitable for some geographic region
       'XQ').  While the precise geographic region is not known outside of
       private agreement, the tag conveys far more information than an
       opaque tag such as "x-somelang" or even "zh-Hans-x-xq" (where the
       'xq' subtag's meaning is entirely opaque).

       However, in some cases content tagged with private use subtags can
       interact with other systems in a different and possibly unsuitable
       manner compared to tags that use opaque, privately defined subtags,
       so the choice of the best approach sometimes depends on the
       particular domain in question.

    5.  IANA Considerations

       This section deals with the processes and requirements necessary for
       IANA to maintain the subtag and extension registries as defined by
       this document and in accordance with the requirements of [RFC5226].

       The impact on the IANA maintainers of the two registries defined by
       this document will be a small increase in the frequency of new
       entries or updates.  IANA also is required to create a new mailing
       list (described below in Section 5.1) to announce registry changes
       and updates.

    5.1.  Language Subtag Registry

       IANA updated the registry using instructions and content provided in
       a companion document [RFC5645].  The criteria and process for
       selecting the updated set of records are described in that document.
       The updated set of records represents no impact on IANA, since the
       work to create it will be performed externally.

       Future work on the Language Subtag Registry includes the following
       activities:

       o  Inserting or replacing whole records.  These records are
          preformatted for IANA by the Language Subtag Reviewer, as
          described in Section 3.3.

       o  Archiving and making publicly available the registration forms.

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       o  Announcing each updated version of the registry on the
          "ietf-languages-announcements@iana.org" mailing list.

       Each registration form sent to IANA contains a single record for
       incorporation into the registry.  The form will be sent to
       <iana@iana.org> by the Language Subtag Reviewer.  It will have a
       subject line indicating whether the enclosed form represents an
       insertion of a new record (indicated by the word "INSERT" in the
       subject line) or a replacement of an existing record (indicated by
       the word "MODIFY" in the subject line).  At no time can a record be
       deleted from the registry.

       IANA will extract the record from the form and place the inserted or
       modified record into the appropriate section of the Language Subtag
       Registry, grouping the records by their 'Type' field.  Inserted
       records can be placed anywhere within the appropriate section; there
       is no guarantee that the registry's records will be placed in any
       particular order except that they will always be grouped by 'Type'.
       Modified records overwrite the record they replace.

       Whenever an entry is created or modified in the registry, the 'File-
       Date' record at the start of the registry is updated to reflect the
       most recent modification date.  The date format SHALL be the "full-
       date" format of [RFC3339].  The date SHALL be the date on which that
       version of the registry was first published by IANA.  There SHALL be
       at most one version of the registry published in a day.  A 'File-
       Date' record is also included in each request to IANA to insert or
       modify records, indicating the acceptance date of the records in the
       request.

       The updated registry file MUST use the UTF-8 character encoding, and
       IANA MUST check the registry file for proper encoding.  Non-ASCII
       characters can be sent to IANA by attaching the registration form to
       the email message or by using various encodings in the mail message
       body (UTF-8 is recommended).  IANA will verify any unclear or
       corrupted characters with the Language Subtag Reviewer prior to
       posting the updated registry.

       IANA will also archive and make publicly available from
       http://www.iana.org each registration form.  Note that multiple
       registrations can pertain to the same record in the registry.

       Developers who are dependent upon the Language Subtag Registry
       sometimes would like to be informed of changes in the registry so
       that they can update their implementations.  When any change is made
       to the Language Subtag Registry, IANA will send an announcement
       message to <ietf-languages-announcements@iana.org> (a self-
       subscribing list to which only IANA can post).

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    5.2.  Extensions Registry

       The Language Tag Extensions Registry can contain at most 35 records,
       and thus changes to this registry are expected to be very infrequent.

       Future work by IANA on the Language Tag Extensions Registry is
       limited to two cases.  First, the IESG MAY request that new records
       be inserted into this registry from time to time.  These requests
       MUST include the record to insert in the exact format described in
       Section 3.7.  In addition, there MAY be occasional requests from the
       maintaining authority for a specific extension to update the contact
       information or URLs in the record.  These requests MUST include the
       complete, updated record.  IANA is not responsible for validating the
       information provided, only that it is properly formatted.  IANA
       SHOULD take reasonable steps to ascertain that the request comes from
       the maintaining authority named in the record present in the
       registry.

    6.  Security Considerations

       Language tags used in content negotiation, like any other information
       exchanged on the Internet, might be a source of concern because they
       might be used to infer the nationality of the sender, and thus
       identify potential targets for surveillance.

       This is a special case of the general problem that anything sent is
       visible to the receiving party and possibly to third parties as well.
       It is useful to be aware that such concerns can exist in some cases.

       The evaluation of the exact magnitude of the threat, and any possible
       countermeasures, is left to each application protocol (see BCP 72
       [RFC3552] for best current practice guidance on security threats and
       defenses).

       The language tag associated with a particular information item is of
       no consequence whatsoever in determining whether that content might
       contain possible homographs.  The fact that a text is tagged as being
       in one language or using a particular script subtag provides no
       assurance whatsoever that it does not contain characters from scripts
       other than the one(s) associated with or specified by that language
       tag.

       Since there is no limit to the number of variant, private use, and
       extension subtags, and consequently no limit on the possible length
       of a tag, implementations need to guard against buffer overflow
       attacks.  See Section 4.4 for details on language tag truncation,
       which can occur as a consequence of defenses against buffer overflow.

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       To prevent denial-of-service attacks, applications SHOULD NOT depend
       on either the Language Subtag Registry or the Language Tag Extensions
       Registry being always accessible.  Additionally, although the
       specification of valid subtags for an extension (see Section 3.7)
       MUST be available over the Internet, implementations SHOULD NOT
       mechanically depend on those sources being always accessible.

       The registries specified in this document are not suitable for
       frequent or real-time access to, or retrieval of, the full registry
       contents.  Most applications do not need registry data at all.  For
       others, being able to validate or canonicalize language tags as of a
       particular registry date will be sufficient, as the registry contents
       change only occasionally.  Changes are announced to
       <ietf-languages-announcements@iana.org>.  This mailing list is
       intended for interested organizations and individuals, not for bulk
       subscription to trigger automatic software updates.  The size of the
       registry makes it unsuitable for automatic software updates.
       Implementers considering integrating the Language Subtag Registry in
       an automatic updating scheme are strongly advised to distribute only
       suitably encoded differences, and only via their own infrastructure
       -- not directly from IANA.

       Changes, or the absence thereof, can also easily be detected by
       looking at the 'File-Date' record at the start of the registry, or by
       using features of the protocol used for downloading, without having
       to download the full registry.  At the time of publication of this
       document, IANA is making the Language Tag Registry available over
       HTTP 1.1.  The proper way to update a local copy of the Language
       Subtag Registry using HTTP 1.1 is to use a conditional GET [RFC2616].

    7.  Character Set Considerations

       The syntax in this document requires that language tags use only the
       characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in most
       character sets, so the composition of language tags shouldn't have
       any character set issues.

       The rendering of text based on the language tag is not addressed
       here.  Historically, some processes have relied on the use of
       character set/encoding information (or other external information) in
       order to infer how a specific string of characters should be
       rendered.  Notably, this applies to language- and culture-specific
       variations of Han ideographs as used in Japanese, Chinese, and
       Korean, where use of, for example, a Japanese character encoding such
       as EUC-JP implies that the text itself is in Japanese.  When language
       tags are applied to spans of text, rendering engines might be able to
       use that information to better select fonts or make other rendering

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       choices, particularly where languages with distinct writing
       traditions use the same characters.

    8.  Changes from RFC 4646

       The main goal for this revision of RFC 4646 was to incorporate two
       new parts of ISO 639 (ISO 639-3 and ISO 639-5) and their attendant
       sets of language codes into the IANA Language Subtag Registry.  This
       permits the identification of many more languages and language
       collections than previously supported.

       The specific changes in this document to meet these goals are:

       o  Defined the incorporation of ISO 639-3 and ISO 639-5 codes for use
          as primary and extended language subtags.  It also permanently
          reserves and disallows the use of additional 'extlang' subtags.
          The changes necessary to achieve this were:

          *  Modified the ABNF comments.

          *  Updated various registration and stability requirements
             sections to reference ISO 639-3 and ISO 639-5 in addition to
             ISO 639-1 and ISO 639-2.

          *  Edited the text to eliminate references to extended language
             subtags where they are no longer used.

          *  Explained the change in the section on extended language
             subtags.

       o  Changed the ABNF related to grandfathered tags.  The irregular
          tags are now listed.  Well-formed grandfathered tags are now
          described by the 'langtag' production, and the 'grandfathered'
          production was removed as a result.  Also: added description of
          both types of grandfathered tags to Section 2.2.8.

       o  Added the paragraph on "collections" to Section 4.1.

       o  Changed the capitalization rules for 'Tag' fields in Section 3.1.

       o  Split Section 3.1 up into subsections.

       o  Modified Section 3.5 to allow 'Suppress-Script' fields to be
          added, modified, or removed via the registration process.  This
          was an erratum from RFC 4646.

       o  Modified examples that used region code 'CS' (formerly Serbia and
          Montenegro) to use 'RS' (Serbia) instead.

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       o  Modified the rules for creating and maintaining record
          'Description' fields to prevent duplicates, including inverted
          duplicates.

       o  Removed the lengthy description of why RFC 4646 was created from
          this section, which also caused the removal of the reference to
          XML Schema.

       o  Modified the text in Section 2.1 to place more emphasis on the
          fact that language tags are not case sensitive.

       o  Replaced the example "fr-Latn-CA" in Section 2.1 with "sr-Latn-RS"
          and "az-Arab-IR" because "fr-Latn-CA" doesn't respect the
          'Suppress-Script' on 'Latn' with 'fr'.

       o  Changed the requirements for well-formedness to make singleton
          repetition checking optional (it is required for validity
          checking) in Section 2.2.9.

       o  Changed the text in Section 2.2.9 referring to grandfathered
          checking to note that the list is now included in the ABNF.

       o  Modified and added text to Section 3.2.  The job description was
          placed first.  A note was added making clear that the Language
          Subtag Reviewer may delegate various non-critical duties,
          including list moderation.  Finally, additional text was added to
          make the appointment process clear and to clarify that decisions
          and performance of the reviewer are appealable.

       o  Added text to Section 3.5 clarifying that the
          ietf-languages@iana.org list is operated by whomever the IESG
          appoints.

       o  Added text to Section 3.1.5 clarifying that the first Description
          in a 'language' record matches the corresponding Reference Name
          for the language in ISO 639-3.

       o  Modified Section 2.2.9 to define classes of conformance related to
          specific tags (formerly 'well-formed' and 'valid' referred to
          implementations).  Notes were added about the removal of 'extlang'
          from the ABNF provided in RFC 4646, allowing for well-formedness
          using this older definition.  Reference to RFC 3066 well-
          formedness was also added.

       o  Added text to the end of Section 3.1.2 noting that future versions
          of this document might add new field types to the registry format
          and recommending that implementations ignore any unrecognized
          fields.

    Phillips & Davis         Best Current Practice                 [Page 74]

    RFC 5646                     Language Tags                September 2009

       o  Added text about what the lack of a 'Suppress-Script' field means
          in a record to Section 3.1.9.

       o  Added text allowing the correction of misspellings and typographic
          errors to Section 3.1.5.

       o  Added text to Section 3.1.8 disallowing 'Prefix' field conflicts
          (such as circular prefix references).

       o  Modified text in Section 3.5 to require the subtag reviewer to
          announce his/her decision (or extension) following the two-week
          period.  Also clarified that any decision or failure to decide can
          be appealed.

       o  Modified text in Section 4.1 to include the (heretofore anecdotal)
          guiding principle of tag choice, and clarifying the non-use of
          script subtags in non-written applications.

       o  Prohibited multiple use of the same variant in a tag (i.e., "de-
          1901-1901").  Previously, this was only a recommendation
          ("SHOULD").

       o  Removed inappropriate [RFC2119] language from the illustration in
          Section 4.4.1.

       o  Replaced the example of deprecating "zh-guoyu" with "zh-
          hakka"->"hak" in Section 4.5, noting that it was this document
          that caused the change.

       o  Replaced the section in Section 4.1 dealing with "mul"/"und" to
          include the subtags 'zxx' and 'mis', as well as the tag
          "i-default".  A normative reference to RFC 2277 was added.

       o  Added text to Section 3.5 clarifying that any modifications of a
          registration request must be sent to the <ietf-languages@iana.org>
          list before submission to IANA.

       o  Changed the ABNF for the record-jar format from using the LWSP
          production to use a folding whitespace production similar to obs-
          FWS in [RFC5234].  This effectively prevents unintentional blank
          lines inside a field.

       o  Clarified and revised text in Sections 3.3, 3.5, and 5.1 to
          clarify that the Language Subtag Reviewer sends the complete
          registration forms to IANA, that IANA extracts the record from the
          form, and that the forms must also be archived separately from the
          registry.

    Phillips & Davis         Best Current Practice                 [Page 75]

    RFC 5646                     Language Tags                September 2009

       o  Added text to Section 5 requiring IANA to send an announcement to
          an ietf-languages-announcements list whenever the registry is
          updated.

       o  Modification of the registry to use UTF-8 as its character
          encoding.  This also entails additional instructions to IANA and
          the Language Subtag Reviewer in the registration process.

       o  Modified the rules in Section 2.2.4 so that "exceptionally
          reserved" ISO 3166-1 codes other than 'UK' were included into the
          registry.  In particular, this allows the code 'EU' (European
          Union) to be used to form language tags or (more commonly) for
          applications that use the registry for region codes to reference
          this subtag.

       o  Modified the IANA considerations section (Section 5) to remove
          unnecessary normative [RFC2119] language.

    9.  References

    9.1.  Normative References

       [ISO15924]       International Organization for Standardization, "ISO
                        15924:2004.  Information and documentation -- Codes
                        for the representation of names of scripts",
                        January 2004.

       [ISO3166-1]      International Organization for Standardization, "ISO
                        3166-1:2006.  Codes for the representation of names
                        of countries and their subdivisions -- Part 1:
                        Country codes", November 2006.

       [ISO639-1]       International Organization for Standardization, "ISO
                        639-1:2002.  Codes for the representation of names
                        of languages -- Part 1: Alpha-2 code", July 2002.

       [ISO639-2]       International Organization for Standardization, "ISO
                        639-2:1998.  Codes for the representation of names
                        of languages -- Part 2: Alpha-3 code", October 1998.

       [ISO639-3]       International Organization for Standardization, "ISO
                        639-3:2007.  Codes for the representation of names
                        of languages - Part 3: Alpha-3 code for
                        comprehensive coverage of languages", February 2007.

    Phillips & Davis         Best Current Practice                 [Page 76]

    RFC 5646                     Language Tags                September 2009

       [ISO639-5]       International Organization for Standardization, "ISO
                        639-5:2008. Codes for the representation of names of
                        languages -- Part 5: Alpha-3 code for language
                        families and groups", May 2008.

       [ISO646]         International Organization for Standardization,
                        "ISO/IEC 646:1991, Information technology -- ISO
                        7-bit coded character set for information
                        interchange.", 1991.

       [RFC2026]        Bradner, S., "The Internet Standards Process --
                        Revision 3", BCP 9, RFC 2026, October 1996.

       [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                        Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2277]        Alvestrand, H., "IETF Policy on Character Sets and
                        Languages", BCP 18, RFC 2277, January 1998.

       [RFC3339]        Klyne, G., Ed. and C. Newman, "Date and Time on the
                        Internet: Timestamps", RFC 3339, July 2002.

       [RFC4647]        Phillips, A. and M. Davis, "Matching of Language
                        Tags", BCP 47, RFC 4647, September 2006.

       [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                        Writing an IANA Considerations Section in RFCs",
                        BCP 26, RFC 5226, May 2008.

       [RFC5234]        Crocker, D. and P. Overell, "Augmented BNF for
                        Syntax Specifications: ABNF", STD 68, RFC 5234,
                        January 2008.

       [SpecialCasing]  The Unicode Consoritum, "Unicode Character Database,
                        Special Casing Properties", March 2008, <http://
                        unicode.org/Public/UNIDATA/SpecialCasing.txt>.

       [UAX14]          Freitag, A., "Unicode Standard Annex #14: Line
                        Breaking Properties", August 2006,
                        <http://www.unicode.org/reports/tr14/>.

       [UN_M.49]        Statistics Division, United Nations, "Standard
                        Country or Area Codes for Statistical Use", Revision
                        4 (United Nations publication, Sales No. 98.XVII.9,
                        June 1999.

    Phillips & Davis         Best Current Practice                 [Page 77]

    RFC 5646                     Language Tags                September 2009

       [Unicode]        Unicode Consortium, "The Unicode Consortium. The
                        Unicode Standard, Version 5.0, (Boston, MA, Addison-
                        Wesley, 2003. ISBN 0-321-49081-0)", January 2007.

    9.2.  Informative References

       [CLDR]           "The Common Locale Data Repository Project",
                        <http://cldr.unicode.org>.

       [RFC1766]        Alvestrand, H., "Tags for the Identification of
                        Languages", RFC 1766, March 1995.

       [RFC2028]        Hovey, R. and S. Bradner, "The Organizations
                        Involved in the IETF Standards Process", BCP 11,
                        RFC 2028, October 1996.

       [RFC2046]        Freed, N. and N. Borenstein, "Multipurpose Internet
                        Mail Extensions (MIME) Part Two: Media Types",
                        RFC 2046, November 1996.

       [RFC2047]        Moore, K., "MIME (Multipurpose Internet Mail
                        Extensions) Part Three: Message Header Extensions
                        for Non-ASCII Text", RFC 2047, November 1996.

       [RFC2231]        Freed, N. and K. Moore, "MIME Parameter Value and
                        Encoded Word Extensions:
                        Character Sets, Languages, and Continuations",
                        RFC 2231, November 1997.

       [RFC2616]        Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                        Masinter, L., Leach, P., and T. Berners-Lee,
                        "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                        June 1999.

       [RFC2781]        Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                        ISO 10646", RFC 2781, February 2000.

       [RFC3066]        Alvestrand, H., "Tags for the Identification of
                        Languages", RFC 3066, January 2001.

       [RFC3282]        Alvestrand, H., "Content Language Headers",
                        RFC 3282, May 2002.

       [RFC3552]        Rescorla, E. and B. Korver, "Guidelines for Writing
                        RFC Text on Security Considerations", BCP 72,
                        RFC 3552, July 2003.

    Phillips & Davis         Best Current Practice                 [Page 78]

    RFC 5646                     Language Tags                September 2009

       [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO
                        10646", STD 63, RFC 3629, November 2003.

       [RFC4645]        Ewell, D., "Initial Language Subtag Registry",
                        RFC 4645, September 2006.

       [RFC4646]        Phillips, A. and M. Davis, "Tags for Identifying
                        Languages", BCP 47, RFC 4646, September 2006.

       [RFC5645]        Ewell, D., Ed., "Update to the Language Subtag
                        Registry", September 2009.

       [UTS35]          Davis, M., "Unicode Technical Standard #35: Locale
                        Data Markup Language (LDML)", December 2007,
                        <http://www.unicode.org/reports/tr35/>.

       [iso639.prin]    ISO 639 Joint Advisory Committee, "ISO 639 Joint
                        Advisory Committee:  Working principles for ISO 639
                        maintenance", March 2000, <http://www.loc.gov/
                        standards/iso639-2/iso639jac_n3r.html>.

       [record-jar]     Raymond, E., "The Art of Unix Programming", 2003,
                        <urn:isbn:0-13-142901-9>.

    Phillips & Davis         Best Current Practice                 [Page 79]

    RFC 5646                     Language Tags                September 2009

    Appendix A.  Examples of Language Tags (Informative)

       Simple language subtag:

          de (German)

          fr (French)

          ja (Japanese)

          i-enochian (example of a grandfathered tag)

       Language subtag plus Script subtag:

          zh-Hant (Chinese written using the Traditional Chinese script)

          zh-Hans (Chinese written using the Simplified Chinese script)

          sr-Cyrl (Serbian written using the Cyrillic script)

          sr-Latn (Serbian written using the Latin script)

       Extended language subtags and their primary language subtag
       counterparts:

          zh-cmn-Hans-CN (Chinese, Mandarin, Simplified script, as used in
          China)

          cmn-Hans-CN (Mandarin Chinese, Simplified script, as used in
          China)

          zh-yue-HK (Chinese, Cantonese, as used in Hong Kong SAR)

          yue-HK (Cantonese Chinese, as used in Hong Kong SAR)

       Language-Script-Region:

          zh-Hans-CN (Chinese written using the Simplified script as used in
          mainland China)

          sr-Latn-RS (Serbian written using the Latin script as used in
          Serbia)

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    RFC 5646                     Language Tags                September 2009

       Language-Variant:

          sl-rozaj (Resian dialect of Slovenian)

          sl-rozaj-biske (San Giorgio dialect of Resian dialect of
          Slovenian)

          sl-nedis (Nadiza dialect of Slovenian)

       Language-Region-Variant:

          de-CH-1901 (German as used in Switzerland using the 1901 variant
          [orthography])

          sl-IT-nedis (Slovenian as used in Italy, Nadiza dialect)

       Language-Script-Region-Variant:

          hy-Latn-IT-arevela (Eastern Armenian written in Latin script, as
          used in Italy)

       Language-Region:

          de-DE (German for Germany)

          en-US (English as used in the United States)

          es-419 (Spanish appropriate for the Latin America and Caribbean
          region using the UN region code)

       Private use subtags:

          de-CH-x-phonebk

          az-Arab-x-AZE-derbend

       Private use registry values:

          x-whatever (private use using the singleton 'x')

          qaa-Qaaa-QM-x-southern (all private tags)

          de-Qaaa (German, with a private script)

          sr-Latn-QM (Serbian, Latin script, private region)

          sr-Qaaa-RS (Serbian, private script, for Serbia)

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       Tags that use extensions (examples ONLY -- extensions MUST be defined
       by revision or update to this document, or by RFC):

          en-US-u-islamcal

          zh-CN-a-myext-x-private

          en-a-myext-b-another

       Some Invalid Tags:

          de-419-DE (two region tags)

          a-DE (use of a single-character subtag in primary position; note
          that there are a few grandfathered tags that start with "i-" that
          are valid)

          ar-a-aaa-b-bbb-a-ccc (two extensions with same single-letter
          prefix)

    Appendix B.  Examples of Registration Forms

       LANGUAGE SUBTAG REGISTRATION FORM

       1. Name of requester: Han Steenwijk
       2. E-mail address of requester: han.steenwijk @ unipd.it
       3. Record Requested:

       Type:        variant
       Subtag:      biske
       Description: The San Giorgio dialect of Resian
       Description: The Bila dialect of Resian
       Prefix:      sl-rozaj
       Comments:    The dialect of San Giorgio/Bila is one of the
          four major local dialects of Resian

       4. Intended meaning of the subtag:

       The local variety of Resian as spoken in San Giorgio/Bila

       5. Reference to published description of the language (book or
       article):

        -- Jan I.N. Baudouin de Courtenay - Opyt fonetiki rez'janskich
       govorov, Varsava - Peterburg: Vende - Kozancikov, 1875.

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    RFC 5646                     Language Tags                September 2009

       LANGUAGE SUBTAG REGISTRATION FORM

       1. Name of requester: Jaska Zedlik
       2. E-mail address of requester: jz53 @ zedlik.com
       3. Record Requested:

       Type:   variant
       Subtag: tarask
       Description: Belarusian in Taraskievica orthography
       Prefix: be
       Comments: The subtag represents Branislau Taraskievic's Belarusian
         orthography as published in "Bielaruski klasycny pravapis" by
         Juras Buslakou, Vincuk Viacorka, Zmicier Sanko, and Zmicier Sauka
         (Vilnia-Miensk 2005).

       4. Intended meaning of the subtag:

       The subtag is intended to represent the Belarusian orthography as
       published in "Bielaruski klasycny pravapis" by Juras Buslakou, Vincuk
       Viacorka, Zmicier Sanko, and Zmicier Sauka (Vilnia-Miensk 2005).

       5. Reference to published description of the language (book or
       article):

       Taraskievic, Branislau. Bielaruskaja gramatyka dla skol. Vilnia: Vyd.
       "Bielaruskaha kamitetu", 1929, 5th edition.

       Buslakou, Juras; Viacorka, Vincuk; Sanko, Zmicier; Sauka, Zmicier.
       Bielaruski klasycny pravapis. Vilnia-Miensk, 2005.

       6. Any other relevant information:

       Belarusian in Taraskievica orthography became widely used, especially
       in Belarusian-speaking Internet segment, but besides this some books
       and newspapers are also printed using this orthography of Belarusian.

    Appendix C.  Acknowledgements

       Any list of contributors is bound to be incomplete; please regard the
       following as only a selection from the group of people who have
       contributed to make this document what it is today.

       The contributors to RFC 4646, RFC 4647, RFC 3066, and RFC 1766, the
       precursors of this document, made enormous contributions directly or
       indirectly to this document and are generally responsible for the
       success of language tags.

    Phillips & Davis         Best Current Practice                 [Page 83]

    RFC 5646                     Language Tags                September 2009

       The following people contributed to this document:

       Stephane Bortzmeyer, Karen Broome, Peter Constable, John Cowan,
       Martin Duerst, Frank Ellerman, Doug Ewell, Deborah Garside, Marion
       Gunn, Alfred Hoenes, Kent Karlsson, Chris Newman, Randy Presuhn,
       Stephen Silver, Shawn Steele, and many, many others.

       Very special thanks must go to Harald Tveit Alvestrand, who
       originated RFCs 1766 and 3066, and without whom this document would
       not have been possible.

       Special thanks go to Michael Everson, who served as the Language Tag
       Reviewer for almost the entire RFC 1766/RFC 3066 period, as well as
       the Language Subtag Reviewer since the adoption of RFC 4646.

       Special thanks also go to Doug Ewell, for his production of the first
       complete subtag registry, his work to support and maintain new
       registrations, and his careful editorship of both RFC 4645 and
       [RFC5645].

    Authors' Addresses

       Addison Phillips (editor)
       Lab126

       EMail: addison@inter-locale.com
       URI:   http://www.inter-locale.com

       Mark Davis (editor)
       Google

       EMail: markdavis@google.com

    Phillips & Davis         Best Current Practice                 [Page 84]

    =========================================================================

    Network Working Group                                   A. Phillips, Ed.
    Request for Comments: 4647                                   Yahoo! Inc.
    BCP: 47                                                    M. Davis, Ed.
    Obsoletes: 3066                                                   Google
    Category: Best Current Practice                           September 2006

                           Matching of Language Tags

    Status of This Memo

       This document specifies an Internet Best Current Practices for the
       Internet Community, and requests discussion and suggestions for
       improvements.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2006).

    Abstract

       This document describes a syntax, called a "language-range", for
       specifying items in a user's list of language preferences.  It also
       describes different mechanisms for comparing and matching these to
       language tags.  Two kinds of matching mechanisms, filtering and
       lookup, are defined.  Filtering produces a (potentially empty) set of
       language tags, whereas lookup produces a single language tag.
       Possible applications include language negotiation or content
       selection.  This document, in combination with RFC 4646, replaces RFC
       3066, which replaced RFC 1766.

    Phillips & Davis         Best Current Practice                  [Page 1]

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    Table of Contents

       1. Introduction ....................................................3
       2. The Language Range ..............................................3
          2.1. Basic Language Range .......................................4
          2.2. Extended Language Range ....................................4
          2.3. The Language Priority List .................................5
       3. Types of Matching ...............................................6
          3.1. Choosing a Matching Scheme .................................6
          3.2. Implementation Considerations ..............................7
          3.3. Filtering ..................................................8
               3.3.1. Basic Filtering .....................................9
               3.3.2. Extended Filtering .................................10
          3.4. Lookup ....................................................12
               3.4.1. Default Values .....................................14
       4. Other Considerations ...........................................15
          4.1. Choosing Language Ranges ..................................15
          4.2. Meaning of Language Tags and Ranges .......................16
          4.3. Considerations for Private-Use Subtags ....................17
          4.4. Length Considerations for Language Ranges .................17
       5. Security Considerations ........................................17
       6. Character Set Considerations ...................................17
       7. References .....................................................18
          7.1. Normative References ......................................18
          7.2. Informative References ....................................18
       Appendix A. Acknowledgements ......................................19

    Phillips & Davis         Best Current Practice                  [Page 2]

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    1.  Introduction

       Human beings on our planet have, past and present, used a number of
       languages.  There are many reasons why one would want to identify the
       language used when presenting or requesting information.

       Applications, protocols, or specifications that use language
       identifiers, such as the language tags defined in [RFC4646],
       sometimes need to match language tags to a user's language
       preferences.

       This document defines a syntax (called a language range (Section 2))
       for specifying items in the user's list of language preferences
       (called a language priority list (Section 2.3)), as well as several
       schemes for selecting or filtering sets of language tags by comparing
       the language tags to the user's preferences.  Applications,
       protocols, or specifications will have varying needs and requirements
       that affect the choice of a suitable matching scheme.

       This document describes how to indicate a user's preferences using
       language ranges, three schemes for matching these ranges to a set of
       language tags, and the various practical considerations that apply to
       implementing and using these schemes.

       This document, in combination with [RFC4646], replaces [RFC3066],
       which replaced [RFC1766].

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

    2.  The Language Range

       Language tags [RFC4646] are used to help identify languages, whether
       spoken, written, signed, or otherwise signaled, for the purpose of
       communication.  Applications, protocols, or specifications that use
       language tags are often faced with the problem of identifying sets of
       content that share certain language attributes.  For example,
       HTTP/1.1 [RFC2616] describes one such mechanism in its discussion of
       the Accept-Language header (Section 14.4), which is used when
       selecting content from servers based on the language of that content.

       It is, thus, useful to have a mechanism for identifying sets of
       language tags that share specific attributes.  This allows users to
       select or filter the language tags based on specific requirements.
       Such an identifier is called a "language range".

    Phillips & Davis         Best Current Practice                  [Page 3]

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       There are different types of language range, whose specific
       attributes vary according to their application.  Language ranges are
       similar to language tags: they consist of a sequence of subtags
       separated by hyphens.  In a language range, each subtag MUST either
       be a sequence of ASCII alphanumeric characters or the single
       character '*' (%x2A, ASTERISK).  The character '*' is a "wildcard"
       that matches any sequence of subtags.  The meaning and uses of
       wildcards vary according to the type of language range.

       Language tags and thus language ranges are to be treated as case-
       insensitive: there exist conventions for the capitalization of some
       of the subtags, but these MUST NOT be taken to carry meaning.
       Matching of language tags to language ranges MUST be done in a case-
       insensitive manner.

    2.1.  Basic Language Range

       A "basic language range" has the same syntax as an [RFC3066] language
       tag or is the single character "*".  The basic language range was
       originally described by HTTP/1.1 [RFC2616] and later [RFC3066].  It
       is defined by the following ABNF [RFC4234]:

       language-range   = (1*8ALPHA *("-" 1*8alphanum)) / "*"
       alphanum         = ALPHA / DIGIT

       A basic language range differs from the language tags defined in
       [RFC4646] only in that there is no requirement that it be "well-
       formed" or be validated against the IANA Language Subtag Registry.
       Such ill-formed ranges will probably not match anything.  Note that
       the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the
       use of digits anywhere in the 'language-range' (see [RFC2616errata]).

    2.2.  Extended Language Range

       Occasionally, users will wish to select a set of language tags based
       on the presence of specific subtags.  An "extended language range"
       describes a user's language preference as an ordered sequence of
       subtags.  For example, a user might wish to select all language tags
       that contain the region subtag 'CH' (Switzerland).  Extended language
       ranges are useful for specifying a particular sequence of subtags
       that appear in the set of matching tags without having to specify all
       of the intervening subtags.

       An extended language range can be represented by the following ABNF:

       extended-language-range = (1*8ALPHA / "*")
                                 *("-" (1*8alphanum / "*"))

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       The wildcard subtag '*' can occur in any position in the extended
       language range, where it matches any sequence of subtags that might
       occur in that position in a language tag.  However, wildcards outside
       the first position are ignored by Extended Filtering (see Section
       3.2.2).  The use or absence of one or more wildcards cannot be taken
       to imply that a certain number of subtags will appear in the matching
       set of language tags.

    2.3.  The Language Priority List

       A user's language preferences will often need to specify more than
       one language range, and thus users often need to specify a
       prioritized list of language ranges in order to best reflect their
       language preferences.  This is especially true for speakers of
       minority languages.  A speaker of Breton in France, for example, can
       specify "br" followed by "fr", meaning that if Breton is available,
       it is preferred, but otherwise French is the best alternative.  It
       can get more complex: a different user might want to fall back from
       Skolt Sami to Northern Sami to Finnish.

       A "language priority list" is a prioritized or weighted list of
       language ranges.  One well-known example of such a list is the
       "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
       14.4) and RFC 3282 [RFC3282].

       The various matching operations described in this document include
       considerations for using a language priority list.  This document
       does not define the syntax for a language priority list; defining
       such a syntax is the responsibility of the protocol, application, or
       specification that uses it.  When given as examples in this document,
       language priority lists will be shown as a quoted sequence of ranges
       separated by commas, like this: "en, fr, zh-Hant" (which is read
       "English before French before Chinese as written in the Traditional
       script").

       A simple list of ranges is considered to be in descending order of
       priority.  Other language priority lists provide "quality weights"
       for the language ranges in order to specify the relative priority of
       the user's language preferences.  An example of this is the use of
       "q" values in the syntax of the "Accept-Language" header (defined in
       [RFC2616], Section 14.4, and [RFC3282]).

    Phillips & Davis         Best Current Practice                  [Page 5]

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    3.  Types of Matching

       Matching language ranges to language tags can be done in many
       different ways.  This section describes three such matching schemes,
       as well as the considerations for choosing between them.  Protocols
       and specifications requiring conformance to this specification MUST
       clearly indicate the particular mechanism used in selecting or
       matching language tags.

       There are two types of matching scheme in this document.  A matching
       scheme that produces zero or more matching language tags is called
       "filtering".  A matching scheme that produces exactly one match for a
       given request is called "lookup".

    3.1.  Choosing a Matching Scheme

       Applications, protocols, and specifications are faced with the
       decision of what type of matching to use.  Sometimes, different
       styles of matching are suited to different kinds of processing within
       a particular application or protocol.

       This document describes three matching schemes:

       1.  Basic Filtering (Section 3.3.1) matches a language priority list
           consisting of basic language ranges (Section 2.1) to sets of
           language tags.

       2.  Extended Filtering (Section 3.3.2) matches a language priority
           list consisting of extended language ranges (Section 2.2) to sets
           of language tags.

       3.  Lookup (Section 3.4) matches a language priority list consisting
           of basic language ranges to sets of language tags to find the one
           exact language tag that best matches the range.

       Filtering can be used to produce a set of results (such as a
       collection of documents) by comparing the user's preferences to a set
       of language tags.  For example, when performing a search, filtering
       can be used to limit the results to items tagged as being in the
       French language.  Filtering can also be used when deciding whether to
       perform a language-sensitive process on some content.  For example, a
       process might cause paragraphs whose language tag matched the
       language range "nl" (Dutch) to be displayed in italics within a
       document.

       Lookup produces the single result that best matches the user's
       preferences from the list of available tags, so it is useful in cases
       in which a single item is required (and for which only a single item

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       can be returned).  For example, if a process were to insert a human-
       readable error message into a protocol header, it might select the
       text based on the user's language priority list.  Since the process
       can return only one item, it is forced to choose a single item and it
       has to return some item, even if none of the content's language tags
       match the language priority list supplied by the user.

    3.2.  Implementation Considerations

       Language tag matching is a tool, and does not by itself specify a
       complete procedure for the use of language tags.  Such procedures are
       intimately tied to the application protocol in which they occur.
       When specifying a protocol operation using matching, the protocol
       MUST specify:

       o  Which type(s) of language tag matching it uses

       o  Whether the operation returns a single result (lookup) or a
          possibly empty set of results (filtering)

       o  For lookup, what the default item is (or the sequence of
          operations or configuration information used to determine the
          default) when no matching tag is found.  For instance, a protocol
          might define the result as failure of the operation, an empty
          value, returning some protocol defined or implementation defined
          default, or returning i-default [RFC2277].

       Applications, protocols, and specifications are not required to
       validate or understand any of the semantics of the language tags or
       ranges or of the subtags in them, nor do they require access to the
       IANA Language Subtag Registry (see Section 3 in [RFC4646]).  This
       simplifies implementation.

       However, designers of applications, protocols, or specifications are
       encouraged to use the information from the IANA Language Subtag
       Registry to support canonicalizing language tags and ranges in order
       to map grandfathered and obsolete tags or subtags into modern
       equivalents.

       Applications, protocols, or specifications that canonicalize ranges
       MUST either perform matching operations with both the canonical and
       original (unmodified) form of the range or MUST also canonicalize
       each tag for the purposes of comparison.

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       Note that canonicalizing language ranges makes certain operations
       impossible.  For example, an implementation that canonicalizes the
       language range "art-lojban" (artificial language, lojban variant) to
       use the more modern "jbo" (Lojban) cannot be used to select just the
       items with the older tag.

       Applications, protocols, or specifications that use basic ranges
       might sometimes receive extended language ranges instead.  An
       application, protocol, or specification MUST choose to a) map
       extended language ranges to basic ranges using the algorithm below,
       b) reject any extended language ranges in the language priority list
       that are not valid basic language ranges, or c) treat each extended
       language range as if it were a basic language range, which will have
       the same result as ignoring them, since these ranges will not match
       any valid language tags.

       An extended language range is mapped to a basic language range as
       follows: if the first subtag is a '*' then the entire range is
       treated as "*", otherwise each wildcard subtag is removed.  For
       example, the extended language range "en-*-US" maps to "en-US"
       (English, United States).

       Applications, protocols, or specifications, in addressing their
       particular requirements, can offer pre-processing or configuration
       options.  For example, an implementation could allow a user to
       associate or map a particular language range to a different value.
       Such a user might wish to associate the language range subtags 'nn'
       (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian) with the more general
       subtag 'no' (Norwegian).  Or perhaps a user would want to associate
       requests for the range "zh-Hans" (Chinese as written in the
       Simplified script) with content bearing the language tag "zh-CN"
       (Chinese as used in China, where the Simplified script is
       predominant).  Documentation on how the ranges or tags are altered,
       prioritized, or compared in the subsequent match in such an
       implementation will assist users in making these types of
       configuration choices.

    3.3.  Filtering

       Filtering is used to select the set of language tags that matches a
       given language priority list.  It is called "filtering" because this
       set might contain no items at all or it might return an arbitrarily
       large number of matching items: as many items as match the language
       priority list, thus "filtering out" the non-matching items.

       In filtering, each language range represents the least specific
       language tag (that is, the language tag with fewest number of
       subtags) that is an acceptable match.  All of the language tags in

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       the matching set of tags will have an equal or greater number of
       subtags than the language range.  Every non-wildcard subtag in the
       language range will appear in every one of the matching language
       tags.  For example, if the language priority list consists of the
       range "de-CH" (German as used in Switzerland), one might see tags
       such as "de-CH-1996" (German as used in Switzerland, orthography of
       1996) but one will never see a tag such as "de" (because the 'CH'
       subtag is missing).

       If the language priority list (see Section 2.3) contains more than
       one range, the content returned is typically ordered in descending
       level of preference, but it MAY be unordered, according to the needs
       of the application or protocol.

       Some examples of applications where filtering might be appropriate
       include:

       o  Applying a style to sections of a document in a particular set of
          languages.

       o  Displaying the set of documents containing a particular set of
          keywords written in a specific set of languages.

       o  Selecting all email items written in a specific set of languages.

       o  Selecting audio files spoken in a particular language.

       Filtering seems to imply that there is a semantic relationship
       between language tags that share the same prefix.  While this is
       often the case, it is not always true: the language tags that match a
       specific language range do not necessarily represent mutually
       intelligible languages.

    3.3.1.  Basic Filtering

       Basic filtering compares basic language ranges to language tags.
       Each basic language range in the language priority list is considered
       in turn, according to priority.  A language range matches a
       particular language tag if, in a case-insensitive comparison, it
       exactly equals the tag, or if it exactly equals a prefix of the tag
       such that the first character following the prefix is "-".  For
       example, the language-range "de-de" (German as used in Germany)
       matches the language tag "de-DE-1996" (German as used in Germany,
       orthography of 1996), but not the language tags "de-Deva" (German as
       written in the Devanagari script) or "de-Latn-DE" (German, Latin
       script, as used in Germany).

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       The special range "*" in a language priority list matches any tag.  A
       protocol that uses language ranges MAY specify additional rules about
       the semantics of "*"; for instance, HTTP/1.1 [RFC2616] specifies that
       the range "*" matches only languages not matched by any other range
       within an "Accept-Language" header.

       Basic filtering is identical to the type of matching described in
       [RFC3066], Section 2.5 (Language-range).

    3.3.2.  Extended Filtering

       Extended filtering compares extended language ranges to language
       tags.  Each extended language range in the language priority list is
       considered in turn, according to priority.  A language range matches
       a particular language tag if each respective list of subtags matches.
       To determine a match:

       1.  Split both the extended language range and the language tag being
           compared into a list of subtags by dividing on the hyphen (%x2D)
           character.  Two subtags match if either they are the same when
           compared case-insensitively or the language range's subtag is the
           wildcard '*'.

       2.  Begin with the first subtag in each list.  If the first subtag in
           the range does not match the first subtag in the tag, the overall
           match fails.  Otherwise, move to the next subtag in both the
           range and the tag.

       3.  While there are more subtags left in the language range's list:

           A.  If the subtag currently being examined in the range is the
               wildcard ('*'), move to the next subtag in the range and
               continue with the loop.

           B.  Else, if there are no more subtags in the language tag's
               list, the match fails.

           C.  Else, if the current subtag in the range's list matches the
               current subtag in the language tag's list, move to the next
               subtag in both lists and continue with the loop.

           D.  Else, if the language tag's subtag is a "singleton" (a single
               letter or digit, which includes the private-use subtag 'x')
               the match fails.

           E.  Else, move to the next subtag in the language tag's list and
               continue with the loop.

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       4.  When the language range's list has no more subtags, the match
           succeeds.

       Subtags not specified, including those at the end of the language
       range, are thus treated as if assigned the wildcard value '*'.  Much
       like basic filtering, extended filtering selects content with
       arbitrarily long tags that share the same initial subtags as the
       language range.  In addition, extended filtering selects language
       tags that contain any intermediate subtags not specified in the
       language range.  For example, the extended language range "de-*-DE"
       (or its synonym "de-DE") matches all of the following tags:

          de-DE (German, as used in Germany)

          de-de (German, as used in Germany)

          de-Latn-DE (Latin script)

          de-Latf-DE (Fraktur variant of Latin script)

          de-DE-x-goethe (private-use subtag)

          de-Latn-DE-1996 (orthography of 1996)

          de-Deva-DE (Devanagari script)

       The same range does not match any of the following tags for the
       reasons shown:

          de (missing 'DE')

          de-x-DE (singleton 'x' occurs before 'DE')

          de-Deva ('Deva' not equal to 'DE')

       Note: [RFC4646] defines each type of subtag (language, script,
       region, and so forth) according to position, size, and content.  This
       means that subtags in a language range can only match specific types
       of subtags in a language tag.  For example, a subtag such as 'Latn'
       is always a script subtag (unless it follows a singleton) while a
       subtag such as 'nedis' can only match the equivalent variant subtag.
       Two-letter subtags in the initial position have a different type
       (language) than two-letter subtags in later positions (region).  This
       is the reason why a wildcard in the extended language range is
       significant in the first position but is ignored in all other
       positions.

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    3.4.  Lookup

       Lookup is used to select the single language tag that best matches
       the language priority list for a given request.  When performing
       lookup, each language range in the language priority list is
       considered in turn, according to priority.  By contrast with
       filtering, each language range represents the most specific tag that
       is an acceptable match.  The first matching tag found, according to
       the user's priority, is considered the closest match and is the item
       returned.  For example, if the language range is "de-ch", a lookup
       operation can produce content with the tags "de" or "de-CH" but never
       content with the tag "de-CH-1996".  If no language tag matches the
       request, the "default" value is returned.

       For example, if an application inserts some dynamic content into a
       document, returning an empty string if there is no exact match is not
       an option.  Instead, the application "falls back" until it finds a
       matching language tag associated with a suitable piece of content to
       insert.  Some applications of lookup include:

       o  Selection of a template containing the text for an automated email
          response.

       o  Selection of an item containing some text for inclusion in a
          particular Web page.

       o  Selection of a string of text for inclusion in an error log.

       o  Selection of an audio file to play as a prompt in a phone system.

       In the lookup scheme, the language range is progressively truncated
       from the end until a matching language tag is located.  Single letter
       or digit subtags (including both the letter 'x', which introduces
       private-use sequences, and the subtags that introduce extensions) are
       removed at the same time as their closest trailing subtag.  For
       example, starting with the range "zh-Hant-CN-x-private1-private2"
       (Chinese, Traditional script, China, two private-use tags) the lookup
       progressively searches for content as shown below:

       Example of a Lookup Fallback Pattern

       Range to match: zh-Hant-CN-x-private1-private2
       1. zh-Hant-CN-x-private1-private2
       2. zh-Hant-CN-x-private1
       3. zh-Hant-CN
       4. zh-Hant
       5. zh
       6. (default)

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       This fallback behavior allows some flexibility in finding a match.
       Without fallback, the default content would be returned immediately
       if exactly matching content is unavailable.  With fallback, a result
       more closely matching the user request can be provided.

       Extensions and unrecognized private-use subtags might be unrelated to
       a particular application of lookup.  Since these subtags come at the
       end of the subtag sequence, they are removed first during the
       fallback process and usually pose no barrier to interoperability.
       However, an implementation MAY remove these from ranges prior to
       performing the lookup (provided the implementation also removes them
       from the tags being compared).  Such modification is internal to the
       implementation and applications, protocols, or specifications SHOULD
       NOT remove or modify subtags in content that they return or forward,
       because this removes information that can be used elsewhere.

       The special language range "*" matches any language tag.  In the
       lookup scheme, this range does not convey enough information by
       itself to determine which language tag is most appropriate, since it
       matches everything.  If the language range "*" is followed by other
       language ranges, it is skipped.  If the language range "*" is the
       only one in the language priority list or if no other language range
       follows, the default value is computed and returned.

       In some cases, the language priority list can contain one or more
       extended language ranges (as, for example, when the same language
       priority list is used as input for both lookup and filtering
       operations).  Wildcard values in an extended language range normally
       match any value that can occur in that position in a language tag.
       Since only one item can be returned for any given lookup request,
       wildcards in a language range have to be processed in a consistent
       manner or the same request will produce widely varying results.
       Applications, protocols, or specifications that accept extended
       language ranges MUST define which item is returned when more than one
       item matches the extended language range.

       For example, an implementation could map the extended language ranges
       to basic ranges.  Another possibility would be for an implementation
       to return the matching tag that is first in ASCII-order.  If the
       language range were "*-CH" ('CH' represents Switzerland) and the set
       of tags included "de-CH" (German as used in Switzerland), "fr-CH"
       (French, Switzerland), and "it-CH" (Italian, Switzerland), then the
       tag "de-CH" would be returned.

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    3.4.1.  Default Values

       Each application, protocol, or specification that uses lookup MUST
       define the defaulting behavior when no tag matches the language
       priority list.  What this action consists of strongly depends on how
       lookup is being applied.  Some examples of defaulting behavior
       include:

       o  return an item with no language tag or an item of a non-linguistic
          nature, such as an image or sound

       o  return a null string as the language tag value, in cases where the
          protocol permits the empty value (see, for example, "xml:lang" in
          [XML10])

       o  return a particular language tag designated for the operation

       o  return the language tag "i-default" (see [RFC2277])

       o  return an error condition or error message

       o  return a list of available languages for the user to select from

       When performing lookup using a language priority list, the
       progressive search MUST process each language range in the list
       before seeking or calculating the default.

       The default value MAY be calculated or include additional searching
       or matching.  Applications, protocols, or specifications can specify
       different ways in which users can specify or override the defaults.

       One common way to provide for a default is to allow a specific
       language range to be set as the default for a specific type of
       request.  If this approach is chosen, this language range MUST be
       treated as if it were appended to the end of the language priority
       list as a whole, rather than after each item in the language priority
       list.  The application, protocol, or specification MUST also define
       the defaulting behavior if that search fails to find a matching tag
       or item.

       For example, if a particular user's language priority list is "fr-FR,
       zh-Hant" (French as used in France followed by Chinese as written in
       the Traditional script) and the program doing the matching had a
       default language range of "ja-JP" (Japanese as used in Japan), then
       the program searches as follows:

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       1. fr-FR
       2. fr
       3. zh-Hant // next language
       4. zh
       5. ja-JP   // now searching for the default content
       6. ja
       7. (implementation defined default)

    4.  Other Considerations

       When working with language ranges and matching schemes, there are
       some additional points that can influence the choice of either.

    4.1.  Choosing Language Ranges

       Users indicate their language preferences via the choice of a
       language range or the list of language ranges in a language priority
       list.  The type of matching affects what the best choice is for a
       user.

       Most matching schemes make no attempt to process the semantic meaning
       of the subtags.  The language range is compared, in a case-
       insensitive manner, to each language tag being matched, using basic
       string processing.  Users SHOULD select language ranges that are
       well-formed, valid language tags according to [RFC4646] (substituting
       wildcards as appropriate in extended language ranges).

       Applications are encouraged to canonicalize language tags and ranges
       by using the Preferred-Value from the IANA Language Subtag Registry
       for tags or subtags that have been deprecated.  If the user is
       working with content that might use the older form, the user might
       want to include both the new and old forms in a language priority
       list.  For example, the tag "art-lojban" is deprecated.  The subtag
       'jbo' is supposed to be used instead, so the user might use it to
       form the language range.  Or the user might include both in a
       language priority list: "jbo, art-lojban".

       Users SHOULD avoid subtags that add no distinguishing value to a
       language range.  When filtering, the fewer the number of subtags that
       appear in the language range, the more content the range will
       probably match, while in lookup unnecessary subtags can cause
       "better", more-specific content to be skipped in favor of less
       specific content.  For example, the range "de-Latn-DE" returns
       content tagged "de" instead of content tagged "de-DE", even though
       the latter is probably a better match.

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       Whether a subtag adds distinguishing value can depend on the context
       of the request.  For example, a user who reads both Simplified and
       Traditional Chinese, but who prefers Simplified, might use the range
       "zh" for filtering (matching all items that user can read) but
       "zh-Hans" for lookup (making sure that user gets the preferred form
       if it's available, but the fallback to "zh" will still work).  On the
       other hand, content in this case ought to be labeled as "zh-Hans" (or
       "zh-Hant" if that applies) for filtering, while for lookup, if there
       is either "zh-Hans" content or "zh-Hant" content, one of them (the
       one considered 'default') also ought to be made available with the
       simple "zh".  Note that the user can create a language priority list
       "zh-Hans, zh" that delivers the best possible results for both
       schemes.  If the user cannot be sure which scheme is being used (or
       if more than one might be applied to a given request), the user
       SHOULD specify the most specific (largest number of subtags) range
       first and then supply shorter prefixes later in the list to ensure
       that filtering returns a complete set of tags.

       Many languages are written predominantly in a single script.  This is
       usually recorded in the Suppress-Script field in that language
       subtag's registry entry.  For these languages, script subtags SHOULD
       NOT be used to form a language range.  Thus, the language range
       "en-Latn" is inappropriate in most cases (because the vast majority
       of English documents are written in the Latin script and thus the
       'en' language subtag has a Suppress-Script field for 'Latn' in the
       registry).

       When working with tags and ranges, note that extensions and most
       private-use subtags are orthogonal to language tag matching, in that
       they specify additional attributes of the text not related to the
       goals of most matching schemes.  Users SHOULD avoid using these
       subtags in language ranges, since they interfere with the selection
       of available content.  When used in language tags (as opposed to
       ranges), these subtags normally do not interfere with filtering
       (Section 3), since they appear at the end of the tag and will match
       all prefixes.  Lookup (Section 3.4) implementations are advised to
       ignore unrecognized private-use and extension subtags when performing
       language tag fallback.

    4.2.  Meaning of Language Tags and Ranges

       Selecting language tags using language ranges requires some
       understanding by users of what they are selecting.  The meanings of
       the various subtags in a language range are identical to their
       meanings in a language tag (see Section 4.2 in [RFC4646]), with the
       addition that the wildcard "*" represents any matching sequence of
       values.

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    4.3.  Considerations for Private-Use Subtags

       Private agreement is necessary between the parties that intend to use
       or exchange language tags that contain private-use subtags.  Great
       caution SHOULD be used in employing private-use subtags in content or
       protocols intended for general use.  Private-use subtags are simply
       useless for information exchange without prior arrangement.

       The value and semantic meaning of private-use tags and of the subtags
       used within such a language tag are not defined.  Matching private-
       use tags using language ranges or extended language ranges can result
       in unpredictable content being returned.

    4.4.  Length Considerations for Language Ranges

       Language ranges are very similar to language tags in terms of content
       and usage.  The same types of restrictions on length that can be
       applied to language tags can also be applied to language ranges.  See
       [RFC4646] Section 4.3 (Length Considerations).

    5.  Security Considerations

       Language ranges used in content negotiation might be used to infer
       the nationality of the sender, and thus identify potential targets
       for surveillance.  In addition, unique or highly unusual language
       ranges or combinations of language ranges might be used to track a
       specific individual's activities.

       This is a special case of the general problem that anything you send
       is visible to the receiving party.  It is useful to be aware that
       such concerns can exist in some cases.

       The evaluation of the exact magnitude of the threat, and any possible
       countermeasures, is left to each application or protocol.

    6.  Character Set Considerations

       Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
       MINUS (%x2D).  Language ranges also use the character ASTERISK
       (%x2A).  These characters are present in most character sets, so
       presentation or exchange of language tags or ranges should not be
       constrained by character set issues.

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    7.  References

    7.1.  Normative References

       [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                       Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2277]       Alvestrand, H., "IETF Policy on Character Sets and
                       Languages", BCP 18, RFC 2277, January 1998.

       [RFC4234]       Crocker, D., Ed. and P. Overell, "Augmented BNF for
                       Syntax Specifications: ABNF", RFC 4234, October 2005.

       [RFC4646]       Phillips, A., Ed., and M. Davis, Ed., "Tags for
                       Identifying Languages", BCP 47, RFC 4646, September
                       2006.

    7.2.  Informative References

       [RFC1766]       Alvestrand, H., "Tags for the Identification of
                       Languages", RFC 1766, March 1995.

       [RFC2616]       Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                       Masinter, L., Leach, P., and T. Berners-Lee,
                       "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                       June 1999.

       [RFC2616errata] IETF, "HTTP/1.1 Specification Errata", October 2004,
                       <http://purl.org/NET/http-errata>.

       [RFC3066]       Alvestrand, H., "Tags for the Identification of
                       Languages", BCP 47, RFC 3066, January 2001.

       [RFC3282]       Alvestrand, H., "Content Language Headers", RFC 3282,
                       May 2002.

       [XML10]         Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
                       and F. Yergeau, "Extensible Markup Language (XML) 1.0
                       (Third Edition)", World Wide Web Consortium
                       Recommendation, February 2004,
                       <http://www.w3.org/TR/REC-xml>.

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    Appendix A.  Acknowledgements

       Any list of contributors is bound to be incomplete; please regard the
       following as only a selection from the group of people who have
       contributed to make this document what it is today.

       The contributors to [RFC1766] and [RFC3066], each of which was a
       precursor to this document, contributed greatly to the development of
       language tag matching, and, in particular, the basic language range
       and the basic matching scheme.  This document was originally part of
       [RFC4646], but was split off before that document's completion.
       Thus, directly or indirectly, those acknowledged in [RFC4646] also
       had a hand in the development of this document, and work done prior
       to the split is acknowledged in that document.

       The following people (in alphabetical order by family name)
       contributed to this document:

       Harald Alvestrand, Stephane Bortzmeyer, Jeremy Carroll, Peter
       Constable, John Cowan, Mark Crispin, Martin Duerst, Frank Ellermann,
       Doug Ewell, Debbie Garside, Marion Gunn, Jon Hanna, Kent Karlsson,
       Erkki Kolehmainen, Jukka Korpela, Ira McDonald, M. Patton, Randy
       Presuhn, Eric van der Poel, Markus Scherer, Misha Wolf, and many,
       many others.

       Very special thanks must go to Harald Tveit Alvestrand, who
       originated RFCs 1766 and 3066, and without whom this document would
       not have been possible.

    Authors' Addresses

       Addison Phillips (Editor)
       Yahoo! Inc.

       EMail: addison@inter-locale.com

       Mark Davis (Editor)
       Google

       EMail: mark.davis@macchiato.com or mark.davis@google.com

    Phillips & Davis         Best Current Practice                 [Page 19]

    RFC 4647               Matching of Language Tags          September 2006

    Full Copyright Statement

       Copyright (C) The Internet Society (2006).

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    Acknowledgement

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    Phillips & Davis         Best Current Practice                 [Page 20]

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