TRANSFORMATIONAL-GENERATIVE GRAMMAR, short form TG. In theoretical LINGUISTICS, a type of generative grammar first advocated by Noam CHOMSKY in Syntactic Structures (1957). Since then, there have been many changes in the descriptive apparatus of TG. Common to all versions is the view that some rules are transformational: that is, they change one structure into another according to such prescribed conventions as moving, inserting, deleting, and replacing items. From an early stage of its history, TG has stipulated two levels of syntactic structure: deep structure (an abstract underlying structure that incorporates all the syntactic information required for the interpretation of a given sentence) and surface structure (a structure that incorporates all the syntactic features of a sentence required to convert the sentence into a spoken or written version). Transformations link deep with surface structure. A typical transformation is the rule for forming questions, which requires that the normal subject—verb order is inverted so that the surface structure of Can I see you later? differs in order of elements from that of I can see you later. The theory postulates that the two sentences have the same order in deep structure, but the question transformation changes the order to that in surface structure. Sentences that are syntactically ambiguous have the same surface structures but different deep structures: for example, the sentence Visiting relatives can be a nuisance is ambiguous in that the subject Visiting relatives may correspond to To visit relatives or to Relatives that visit. The ambiguity is dissolved if the modal verb can is omitted, since the clausal subject requires a singular verb (Visiting relatives is a nuisance), whereas the phrasal subject requires the plural (Visiting relatives are a nuisance).