The basic idea is that of top-down design - beginning with a very abstract representation, and elaborating that, in step-by-step fashion, into a complete and detailed representation. The basic operations are real-time parametric variation of designs (using the mouse and slide bar) and substitution of objects. Essentially, then, a knowledge-base in Topdown implements a kind of parametric shape grammar.

The main applications of Topdown are in introductory teaching of CAD, and (since it provides a very quick and easy way for a user to develop detailed geometric models) to provide a uniform front-end for a variety of different applications. The shell, and some example knowledge-bases, are publicly available.

This paper discusses the principles of the Topdown Shell, the implementation of knowledge bases within it, and a variety of practical design applications.