Summary

An isogloss is defined as a line that divides two areas in which a single feature has distinct values. The features apply to all linguistic levels and can be synchronic or diachronic. In Romance studies, isoglosses are generally traced on the basis of phonological and diachronic features. Very early on it was observed that, depending on the feature selected, different zones were outlined (noncoincidence of isoglosses). From this arose skepticism with respect to the possibility of delineating dialect groups. It was noted, however, that isoglosses often follow a trend that is at least parallel, if not coinciding (isogloss bundles). Research has therefore recognized the existence of dialect boundaries and has continued to investigate the correlation between these boundaries and physical or cultural ones.

The isogloss is a problematic instrument for several aspects: It imposes a two-dimensional representation of linguistic reality that leaves no space for vertical variation (diastratic, diaphasic). Moreover, varieties do not always demonstrate a juxtaposition that can be represented by an isogloss (e.g., linguistic enclaves or bilingual areas). A further question is whether it is necessary to establish a hierarchy of isoglosses (phonological, morphological, lexical, etc.). Despite these issues, the isogloss remains a fundamental instrument for linguistic geography.

The major isogloss bundles distinguish dialect groups: Sardinian, Romanian, Galician-Portuguese, Astur-Leonese, Castilian, Navarro-Aragonese, Catalan, Gascon, Occitanic, French, Alpine Romance, Cisalpine Romance, and Italian. For each bundle, the article attempts to determine whether and how it has changed over time, and what the possible cultural correlations of this might be.