ABSTRACT This paper critically examines the construction of ‘supradialects’ (BCMS narječje) in Central South Slavic (CSS) dialectology. The concept is unique because it is usually placed in the hierarchy of varieties above ‘dialect’ and below ‘language’, also because supradialects are indicated as the ‘dialectal bases’ of Bosnian, Croatian, Montenegrin and Serbian (BCMS) languages (and their predecessor, Serbo-Croatian). Despite that, the definition of supradialects in CSS literature is paradoxical: they are not fully described varieties, but sets of old isoglosses, reconstructed by dialectologists by comparing remote village speech varieties to *proto-Slavic. This poses the question of how they can act as a “base” of a contemporary standard language. To explore this issue, I analyse the CSS dialectological literature of the past two centuries and inspect the metalanguage used to define and construct supradialects. I also illustrate how the use of the “base” metaphor in authoritative publications on BCMS languages creates an understanding of supradialects as homogenous, contemporary varieties, concealing their (re)constructed nature. Theoretically, the paper offers and alternative perspective on dialects as discursive constructs in their own terms rather than by-products of linguistic standardization, as they are most often treated in sociolinguistics.