X标杆理论是语言学分支句法学重要术语。 X-bar theory（X标杆理论）refers to NP/VP/AP/PP all must contain one obligatory word, we call XP. This means that XP must contain X, where XP stands respectively for NP/VP/AP/PP and X stands for N/V/A/P.

The rule modified as: XP → (Specifier)X(Complement).

X 标杆理论是将各种短语结构规则高度抽象地综合成单一的X标杆规则理论。根据该理论，短语由指示语、核心词和补语组成，其中指示语和补语可有可无，但核心词却不可缺少。

主要词类：

Noun (N) 名词：student

Verb (V) 动词：like

Adjective (Adj) 形容词：tall

Adverb (Adv) 副词：loudly

附加:phrase(P)短语：如动词短语Verb phrase（VP)

次要词类：

Determiner (Det) 限定词：the, a, this, his

Auxiliary (Aux) 助动词：can, do, will

Preposition (Prep) 介词：in, at, over

Pronoun (Pron) 代词：he, she, us, mine

Conjunction (Conj) 连词：and, or, but, while

Interjection (Int) 感叹词：oh, ah, eh