For a detailed presentation of X-bar Theory and examples for all major syntactic phrases, see the section on X-bar Theory in the General Linguistics area.
In X-bar Theory syntactic entities are projected in the form of tree structures. In Government and Binding, one goal is to find similarities among the different categories of lexical phrases in order to project them in the same way. Rather than giving a different structure to every lexical phrase, there are just two basic rules which cover all lexical categories.
Phrase Structure Rules (for any lexical category, X = head)
X-bar Theory specifies the combination of a lexical head with its complements, modifiers, and specifier. For each grammatical function there is a schematic rule.
- For specifiers: XP → Specifier X'
- For complements: X'→ X Complements (=YP)
These rules generate the following tree configurations.
(Literature: cf. Black: A step-by-step introduction to GB)
Different Versions of X-bar Theory
The rules do not mention adjuncts. In this section we assume the following rules for adjuncts:
- For adjuncts:
- post-head modifiers: X'→ X' Adjunct (=WP)
- pre-head modifiers: X'→ Adjunct (=WP) X'
All syntactic theories presented here adopt syntactic structures based on the notions of head, complement, adjunct, and specifier. In this section, we adopt the version shown above.
The next tree shows another version of X-bar Theory. In contrast to the rules stated above, it assumes that adjuncts attach to XP categories, not to X' categories.
It can be seen, though, that the distinction among heads, complements, modifiers, and specifiers is encoded in the structure independent of specific assumptions. Also note that adjuncts do not change the bar-level in any version of X-bar Theory.