It is well documented that the study of differences in grammaticality contrasts across the world's languages has implications for the synchronic study of preferential/frequency contrasts within a single language. Our paper extends this observation, arguing that the cross-linguistic study of both grammaticality and frequency contrasts can be crucial to the proper characterization of patterns of diachronic change. As an illustration of this proposal, we investigate patterns of synchronic and diachronic variation in the use of postverbal negative quantifiers (e.g., nothing, nobody, no book, etc., as in, I know nothing) versus negative polarity items under negation (e.g., not … anything, not … anybody, not … any book, etc., as in, I don't know anything) in English. We show how a detailed comparison with similar patterns found elsewhere in closely related languages can give us a better understanding of which linguistic factors condition the use of these different kinds of indefinites in Modern Spoken English and a new perspective on a well-studied proposed change in progress in the English quantificational system.