The issue of decidability in relation to shape grammars is considered here. Decidability concerns, first, the identification of different types of grammars and, second, the answerability or solvability of questions about these types of grammars. In this paper, the first of these two topics is examined. Six different types of shape grammars are defined by considering different kinds of restrictions on rule format and rule ordering. The effects that these different restrictions have on the generative power, practicality, pedagogical value, and other characteristics of a shape grammar are discussed. In a subsequent paper, “Shape grammars: five questions” (Knight, 1998), the answerabilities of various questions about the types of shape grammars outlined here are explored. The decidability issues addressed in this paper and the subsequent one are key to the practical use of shape grammars in design projects where specific goals and constraints need to be satisfied.