High Quality Content by WIKIPEDIA articles! X-bar theory is a component of linguistic theory which attempts to identify syntactic features common to all languages. IT claims that among their phrasal categories, all languages share certain structural similarities, including one known as the "X-bar", which does not appear in traditional phrase structure rules. X-bar theory was first proposed by Chomsky (1970) and further developed by Jackendoff (1977). THe letter X is used to signify an arbitrary lexical category; when analyzing a specific utterance, specific categories are assigned. THus, the X may become an N for noun, a V for verb, an A for adjective, or a P for preposition.