A REXX program is made up of one or more executable units. Directives separate these units, which themselves are REXX programs. REXX processes all directives first to set up any classes, methods, or routines needed by the program. Then it runs any code that exists before the first directive. The first directive in a program marks the end of the executable part of the program. A directive is a kind of clause that begins with a double-colon (::) and is non-executable (a directive cannot appear in the expression of an INTERPRET instruction, for example).