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This article is an introduction to many of the interesting features of PegaSys, an experimental system that encourages and facilitates extensive use of graphical images as formal, machine- processable documentation. Unlike most other systems that use graphics to describe programs, the main purpose of PegaSys is to facilitate the explanation of program designs. What is particularly interesting about PegaSys is its ability to: (1) check whether pictures are syntactically meaningful, (2) enforce design rules throughout the hierarchical decomposition of a design, and (3) determine whether a program meets its pictorial documentation. Much of the power of PegaSys stems from its ability to represent and reason about different kinds of pictures within a single logical framework. Excerpts from a working session with PegaSys are used to illustrate the basic style of interaction as well as the three PegaSys capabilities.