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 then a Zone J has been set up as another possible genealogical group bordering the
Great Lakes.