What is an isogloss?

This short contribution discusses the term and concept of isogloss: the space where a linguistic phenomenon exists or, by metonymic extension, the line that separates a particular linguistic phenomenon from another. Although the concept is currently used as an established and canonical term in linguistics, it is reasonable to explore its scope and the real sense that it carries, as well as to consider its origins and potential ambiguity in use. The main issue with the concept is that it seems to be theoretically necessary yet at the same time is not empirically attestable, which runs contrary to essential epistemological principles. We will seek to show that a) there is no contradiction between the existence of isoglosses as discrete units and the continuity of phenomena on the empirical level, b) the supposed end of the era of the diatopic anchorage of language in times of modern migration is a myth, c) that isoglosses should not be considered as established lines but rather as projections resulting from processes of individual constructions of spaces, and finally, d) that a central task of dialectology and of historical linguistics consists of the identification of the multi-layered superposition of different isoglosses that reflects the complex history of a language or variety.