In many of the first English headed which-relatives, which has an NP complement. Using distributional tests grounded in contrasts revealed by research in formal semantics, we demonstrate that the presence of an NP complement forces a nonrestrictive interpretation of the relative, while ‘bare’ which-relatives may be restrictive or nonrestrictive. We situate this finding in relation to both the formal semantics of relative clauses, and the history of wh-relatives in English.