Specifier (linguistics) in the context of Noam Chomsky

In linguistics, X-bar theory is a model of phrase structure and a theory of syntactic category formation that proposes a universal schema for how phrases are organized. It suggests that all phrases share a common underlying structure, regardless of their specific category (noun phrase, verb phrase, etc.). This structure, known as the X-bar schema, is based on the idea that every phrase (XP, X phrase) has a head, which determines the type (syntactic category) of the phrase (X).

The theory 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 aimed to simplify and generalize the rules of grammar, addressing limitations of earlier phrase structure models. X-bar theory was an important step forward because it simplified the description of sentence structure. Earlier approaches needed many phrase structure rules, which went against the idea of a simple, underlying system for language. X-bar theory offered a more elegant and economical solution, aligned with the thesis of generative grammar.