This chapter investigates to what extent diatopic variation, which can be at best studied through dialects and non-standard varieties, constitutes a tool to reconstruct and understand diachronic change. By examining the relationship between synchronic diatopic and diachronic variation with respect to abstract trajectories of linguistic change in two case studies, that is, phonological variation in the forms of the article in Sicilian and the morphosyntactic distribution of relativizers across Italo-Romance dialects, we aim to test: (i) how synchronic diatopic data map diachronic data in terms of range of variation; and (ii) to what extent synchronic diatopic variation can refine diachronic claims. We demonstrate that there is a clear coincidence between dialectal and diachronic micro-variation which can fruitfully be exploited to better understand linguistic variation at all levels of grammar.