A modeling scheme is presented which provides a medium for the rigorous, formal, and abstract specification of large-scale software system components. The scheme allows the description of component behavior without revealing or requiring the description of a component's internal operation. Both collections of sequential processes and the data objects which they share may be described. The scheme is of particular value during the early stages of software system design, when the system's modules are being delineated and their interactions designed, and when rigorous, well-defined specification of undesigned components allows formal and informal arguments concerning the design's correctness to be formulated.