Guthrie classification of Bantu languages

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.).