The structural make-up of the English NP is a matter of longstanding debate. In this paper, a closer look is taken at a notably intricate part of it, viz. the determiner, and more specifically where it fuzzily borders on the adjective. It will be argued that any attempt to resolve the indeterminacy issues associated with this boundary needs to take the diachrony of NP syntax as a vantage point. More specifically, it will be claimed that what are often conveniently but somewhat misleadingly called postdeterminers are in fact elements undergoing a diachronic transition from adjective slotfiller to determiner slotfiller. The postdeterminer slot is hence not a stable position. 1 I wish to thank Lobke Ghesquie`re, Joop van der Horst, the participants of the Sheffield workshop 'History and Structure in the English Noun Phrase', and the anonymous reviewers for helpful comments on earlier drafts of this paper.2 PeripheralM stands for peripheral modifier, Det for determiner, Adj for adjective, and N for noun.