A set theoretic formalization of a transformational theory in the spirit of Chomsky’s LSLT is presented. The theory differs from Chomsky’s, and more markedly from most current theories, in the extent to which restrictions are imposed on descriptive power. Many well-justified and linguistically significant limitations on structural description and structural change are embodied in the present formalization. We give particular attention to the constructs Phrase Marker and Transformational Cycle providing modifications which offer increases in both simplicity and explanatory power.