For other "bar" connected with formal languages see Bar (computer science)

X-bar

Bar (diacritic)

A bar or stroke is a modification consisting of a line drawn through a grapheme. It may be used as a diacritic to derive new letters from old ones, or simply as an addition to make a grapheme more distinct from others....

 theory is a component of linguistic

Linguistics

Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

 theory which attempts to identify syntactic features presumably common to all those human languages that fit in a presupposed (1965) framework. It claims that among their phrasal categories, all those languages share certain structural similarities, including one known as the "X-bar", which does not appear in traditional phrase structure rules

Phrase structure rules

Phrase-structure rules are a way to describe a given language's syntax. They are used to break down a natural language sentence into its constituent parts namely phrasal categories and lexical categories...

 for English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 or other natural languages.