The RFC 3066 coding scheme is constructed from a primary subtag component encoded using the language codes of ISO 639, plus two codes for extensions for languages not represented in ISO 639. The code optionally includes a second subtag component encoded using the two letter country codes of ISO 3166, or a language code extension registered by the Internet Assigned Names Authority.

RFC 3066 may be obtained at http://www.ietf.org/rfc/rfc3066.txt. RFC 3066 obsoletes RFC 1766.

ISO 639 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 http://www.iso.ch/iso/en/prods-services/iso3166ma/02iso-3166-code-lists/index.html

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

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, and several IANA-registered language code extensions, using Coding Scheme Designator IANARFC1766. RFC 3066 identifies a preference for the ISO 639-1 two letter codes to the ISO 639-2 three letter codes, and the ISO 639-2/T (terminology) subset to the ISO 639-2/B (bibliographic) subset.

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.