X-bar theory

X-bar theory is a linguistic theory which attempts to identify syntactic features common to all languages. The theory claims that there are certain structural similarities among all phrasal categories of all languages. The letter X is used to signify an arbitrary lexical category for the head of the phrase (instead of, for example, N for noun or V for verb) in order to allow a generic description.

X-bar theory adds an additional levels of hierarchy to more traditional phrase structure rules using the phrasal constituent called an X-bar.

The head, X, is said to combine with a complement to form an X-bar, and an adjunct is said to combine with an X-bar to form another X-bar. Finally, an X-bar is said to combined with a specifier to form the top level of the phrase, the X Phrase (or XP). [In this way, complements are distinguished from adjuncts by the fact that a complement has an X as a sister, whereas an adjunct has X-bar as a sister.]

These rules can be formalized as follows:

XP --> (specifier), X-bar An X Phrase consists of an optional specifier and an X-bar, in either order.
(X-bar --> X-bar, adjunct) An X-bar may consist of an X-bar and an adjunct, in either order.
X-bar --> X, (complement...) An X-bar consists of an X and any number of complements, in either order.

Note, the adjuct rule is itself optional.

Examples

We can see the syntactic structure of the verb phrase He studies linguistics at the university. The head of the VP is the verb studies which forms a V-bar with the complement linguistics. The V-bar studies linguistics then forms another V-bar with the adjunct at the university. Finally, a VP (verb phrase) is formed from the specifier NP he and the V-bar studies linguistics at the university. The syntactic structure of the noun phrase The student of linguistics at the university is almost exactly parallel. The head is the noun student, which forms an N-bar with the complement of linguistics, and the N-bar student of linguistics then forms an N-bar with the adjunct at the university. This last N-bar then forms an NP along with the determiner the, which acts as a specifier.