The RFC 4646 coding scheme is constructed from a primary subtag component encoded using the language codes of ISO 639, plus codes for extensions for languages not represented in ISO 639. The code optionally includes additional subtag components, for scripts encoded using the four letter codes of ISO 15924, and for regions encoded using the two letter country codes of ISO 3166.

RFC 4646 may be obtained at http://www.ietf.org/rfc/rfc4646.txt. RFC 4646 obsoletes RFC 3066 and RFC 1766, but is forward compatible with those specifications.

ISO 639 codes may be obtained at http://www.loc.gov/standards/iso639-2/langhome.html.

The two letter country codes of ISO 3166 may be obtained at https://www.iso.org/obp/ui/#search/code/

IANA language tag registrations may be obtained at http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

In previous editions of the Standard, this Context Group formerly included the three letter language codes of ISO 639-2/B, using Coding Scheme Designator ISO639_2, or the language codes of RFC 3066, using Coding Scheme Designator RFC3066, and several IANA-registered language code extensions, using Coding Scheme Designator IANARFC1766.

In previous editions of the Standard, this Context Group provided only language identifiers, with national or regional variant identified in a separate attribute or Content Item.