X-bar theory

In linguistics, X-bar theory is a model of phrase-structure grammar and a theory of syntactic category formation that was first proposed by Noam Chomsky in 1970 reformulating the ideas of Zellig Harris (1951), and further developed by Ray Jackendoff (1974, 1977a, 1977b), along the lines of the theory of generative grammar put forth in the 1950s by Chomsky. It attempts to capture the structure of phrasal categories with a single uniform structure called the X-bar schema, basing itself on the assumption that any phrase in natural language is an XP (X phrase) that is headed by a given syntactic category X. It played a significant role in resolving issues that phrase structure rules had, representative of which is the proliferation of grammatical rules, which is against the thesis of generative grammar.

X-bar theory was incorporated into both transformational and nontransformational theories of syntax, including government and binding theory (GB), generalized phrase structure grammar (GPSG), lexical-functional grammar (LFG), and head-driven phrase structure grammar (HPSG). Although recent work in the minimalist program has largely abandoned X-bar schemata in favor of bare phrase structure approaches, the theory's central assumptions are still valid in different forms and terms in many theories of minimalist syntax.