A routine is a modular unit of a program. In Tcl, one routine is associated with each command. The official documentation currently uses the term "command procedure" in place of the term "routine". In order to evaluate a command, Tcl uses the first word of a command to find and execute the associated routine. Many built-in commands are implemented as C functions. In the case of a procedure, the primary function of the routine is to evaluate the body of the procedure.