xrefer - Model In Chomsky's classic transformational model of grammar (Aspects of the Theory of Syntax, 1965), a few syntactic rules in the base of the grammar provided a syntactic deep structure which was then elaborated by processes known as transformations in order to produce a surface structure. Semantics or meaning was dependent on the deep structure, while phonology or sound was dependent on the surface structure. More recently, in Knowledge of Language (1986), Chomsky has envisaged a model of grammar which is less obviously directional, and which, as with a computer program, is composed of a series of modules, each of which is fairly simple in its general workings, but which becomes complex as it interacts with other modules.