 To distinguish language variants for countries, regions, writing systems etc., IETF language tags combine subtags from other standards such as ISO 639, ISO 15924, ISO 3166-1, and UN M.49.The tag structure has been standardized by the Internet Engineering Task Force (IETF) in "Best Current Practice (BCP) 47"; the subtags are maintained by the "IANA Language Subtag Registry".IETF language tags are used by computing standards such as HTTP,, HTML, XML, and PNG.