The 250 or so "Narrow Bantu languages" are conventionally divided up into geographic zones first proposed by Malcolm Guthrie (1967–1971). These were assigned letters A–S and divided into decades (groups A10, A20, etc.); individual languages were assigned unit numbers (A11, A12, etc.), and dialects further subdivided (A11a, A11b, etc.). This coding system has become the standard for identifying Bantu languages; it was the only practical way to distinguish many ambiguously named languages before the introduction of ISO 639-3 coding, and it continues to be widely used. Only Guthrie's Zone S is (sometimes) considered to be a genealogical group. Since Guthrie's time a Zone J (made of languages formerly classified in groups D and E) has been set up as another possible genealogical group borderin