X-bar theory proposes that all phrases in all languages share a core hierarchical structure with three levels: a maximal projection (X), an intermediate projection (X'), and a head projection (X'). It aims to define phrase structure rules that capture commonalities across categories and distinguish complements from adjuncts. The theory proposes a set of generalized rules using variables that can represent different syntactic categories. Evidence for intermediate projections comes from constituency tests and replacement operations like one-replacement and do-so replacement. X-bar theory provides a framework for exploring cross-linguistic similarities and differences in syntactic structure.
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