In this article, we present a system for the representation of the configurations of the thumb in the hand configurations of signed languages and for the interactions of the thumb with the four fingers proper. The configuration of the thumb is described as a componential combination of the descriptions of thumb opposition, abduction of the CM joint, and extension of the MCP and DIP joints. Interaction with the other fingers is described as a relationship between specific surfaces of the thumb and specific surfaces of the fingers involved. (Contains 14 figures, 4 tables and 3 notes.)