Current interests in software engineering have posed serious questions about the evolution of programs and languages. Computer programs are not simply collections of statements; they involve specific structural relationships between the programelements. Program structure has been discussed as being an important influence on the ease with which programs can be constructed, verified, understood, and changed. The discipline of "structured programming" has been developed because computer scientists have sought to better control and understand the programming process. Program graphs have been used as a vehicle to focus attention on the structure of a program. In this paper a systematic methodology for partitioning a program graph (digraph) to highlight the relationships between program elements is. introduced along with an attendant notation. This notation is described in purely mathematical terms in the first section, and then the programming-related implications of this approach are addressed in the second section.