A phase is a syntactic domain first hypothesised by Noam Chomsky in 1998.Chomsky, Noam (1998). "Minimalist Inquiries: The Framework" MIT Occasional Papers in Linguistics 15. Republished in 2000 in R. Martin, D. Michaels, & J. Uriagereka (eds.). Step By Step: Essays In Syntax in Honor of Howard Lasnik. 89-155. MIT Press. A simple sentence is decomposed into two phases, CP and vP (see X-bar theory). Movement of a constituent out of a phase is (in the general case) only permitted if the constituent has first moved to the left edge of the phase. This condition is specified in the Phase Impenetrability Constraint, which has been variously formulated within the literature.

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