The incorporation of a rule of V MOVEMENT into our description of English Syntax turns out to have fundamental theoretical implications for our overall Theory of Grammar: it means that we are no longer able to posit that the syntactic structure of a sentence can be described in terms of a single Phrase-marker representing its S-structure. For, the postulation of a rule of V-MOVEMENT means that we must recognise at least two different levels of structure in our Theory of Grammar — namely, a level of D-structure (formerly known as ‘Deep Structureʼ) which serves as input to the rule, and a separate level of S-structure which is formed by application of the rule.