ISO 639 is a standardized nomenclature used to classify all known languages. Each language is assigned a two-letter (639-1) and three-letter (639-2 and 639-3), lowercase abbreviation, amended in later versions of the nomenclature. The system is highly useful for linguists and ethnographers to categorize the languages spoken on a regional basis, and to compute analysis in the field of lexicostatistics. ISO 639 has five code lists.