It is suggested that expert systems storing the design knowledge of particular offices in terms of stylistic and construction practice provide a means to take considerably more advantage of information technology than currently. The form of the knowledge stored by such expert systems is a building representation in the form of rules stating how components are placed in three-dimensional space relative to each other. By describing how Frank Lloyd Wright designed his Usonian houses it is demonstrated that the proposed approach is very much in the spirit of distinguished architectural practice. To illustrate this idea, a system for assembling three-dimensional architectural details is presented with particular emphasis on the nature of the rules and the form of the building components created by the rules to assemble typical details. The nature of the rules, which are a three-dimensional adaptation of Stiny's shape grammars, is described. In particular, it is shown how the rules themselves are structured into different classes, what the nature of these classes is and how specific rules can be obtained from more general rules. The rules embody a firm's collective design experience in detailing. As a conclusion, an overview is given of architectural practice using rule-based representations.