Shape grammars: six types

The issue of decidability in relation to shape grammars is considered here. Decidability concerns, first, the identification of different types of gra mmars and, second, the answerability or solvability of questions about thes e types of grammars. In this paper, the first of these two topics is examin ed. Six different types of shape grammars are defined by considering differ ent kinds of restrictions on rule format and rule ordering. The effects tha t these different restrictions have on the generative power, practicality, pedagogical value, and other characteristics of a shape grammar are discuss ed. 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 pape r 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.