and their classification
The original reference is the classification by Malcolm Guthrie (1967–1971), which organises languages into geographical zones designated by letters (A to S, excluding I, O and Q), subdivided into numbered groups. Ex:
A74aBulu — Zone A, Group 70, Language 4, Variant a
This classification has been revised several times (Tervuren, Maho). It is intended to be continually expanded.