 After syntactic structures, the nature of linguistic research began to change, and in 1965 Chomsky developed what is now known as the standard theory of transformational generative grammar, in which the basic formulations presented in syntactic structures underwent considerable revision. In aspects he specified the fundamental distinction between competence and performance, looking at a grammar of a language as a description of the ideal speaker's intrinsic competence, which in turn is a system of generative processes. These processes start in the syntactic component, which consists of a base and a transformational component. The base consists of a categorical subcomponent and a lexicon. It generates a deep structure, which receives a semantic interpretation in the new semantic component. By means of transformational rules, the deep structure is mapped into a surface structure, which is then given a phonetic interpretation by the rules of the phonological component. The addition of a semantic component to the grammar was the most important conceptual change since syntactic structures. It is reminiscent to Katz and Postl's suggestions in 1964. Among the more technical innovations are the use of recursive phrase structure rules and the introduction of syntactic features in lexical entries, the system of subcategorization.