Subjacency

Subjacency is a concept in formal linguistics, particularly within generative grammar, that describes a condition on the movement of syntactic constituents. Proposed by Noam Chomsky in the 1970s, the subjacency condition aims to explain why certain types of grammatical transformations are permissible while others are not. It is a constraint that restricts how far a constituent can move across certain types of phrasal boundaries.

The core idea of subjacency is that a constituent cannot move out of more than one "subjacency barrier" in a single operation. Subjacency barriers are typically defined as maximal projections of certain phrasal categories, such as NP (noun phrase), PP (prepositional phrase), and S' (complementizer phrase). Essentially, if a constituent has to cross two or more of these barrier types to reach its surface position, the movement is blocked.

For example, in English, a wh-phrase (like "what") can move from its original position to the front of a sentence to form a question. However, if this movement requires it to cross two embedded clauses, for instance, it might violate the subjacency condition. The exact definition of subjacency barriers and the application of the principle have been subject to refinement and debate within linguistic theory over time, with various theories proposing alternative or more complex constraint systems. Despite these developments, subjacency remains a significant concept for understanding the limitations on syntactic movement.