Subjacency

This term was first defined by Noam Chomsky in 1973 and constitutes the main concept of the Government and Binding Theory.

This principle states that no movement can move an element over more than one bounding node at a time.

The notion of bounding was first observed in the early generative grammar by, for instance, John R. Ross (1967).

As the specifier of CP position is empty in (1), the wh-element may use it as an escape hatch before moving further.

In the example (2), on the other hand, the specifier of CP position is already taken and the wh-element moves over two AgrP at a time, violating the Subjacency condition and yielding the ungrammatical sentence.