In minimalist theory, the requirement that substitution operations in overt syntax always extend their target is called Extension requirement. This requirement is met iff the designated position E for GT in overt syntax is external to the phrase-marker K, i.e. extends K, unless E is an adjunct position. In effect, GT can only insert material into the highest specifier position of K*.
- Chomsky, N. 1993. A Minimalist Program for Linguistic Theory, MIT occasional papers in linguistics, 1-67. Reprinted in: Chomsky (1995).