X-bar theory is a component of 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 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 combine 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 that the adjunct 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.

(Note that this is only one of many possible analyses of this sentence compatible with X-bar theory).

Though X-bar clauses may seem arbitrary and unneeded, their existence can be confirmed by substitution. To the above sentence, "He studies linguistics at the university," someone could reply, "Oh, she does, too." The word "does," here, stands for the entire Vbar phrase, "studies linguistics at the university", thus implying the existence of this phrase as a complete unit of the whole sentence. In other words: if the V-bar phrase above was not defined as such, the sentence would have three separate phrases directly underneath S: the Noun phrase, the Verb phrase, and the Prepositional phrase. To substitute for two of them, together, as shown, implies that these two, together, make up one phrase within the sentence.

For typographic simplicity the form X′ is often used in place of X-bar, since overbars are often difficult to typeset and are unavailable in many fonts. XP, which can also be labeled X-bar-bar (that is, with two overbars), is also seen as X″ instead. In English, linguists still pronounce the prime symbol as "bar" and the double prime as "double bar" in the context of X-bar theory for continuity with the original formulation.

See also

• minimalist syntax
• linguistics
• phrase structure rules
• syntax
• X-Bar Morphosyntax