Isogloss

Isogloss in the Faroe Islands.

An isogloss is an imaginary line (isoline) that separates two geographical areas that are distinguished by a specific dialectal feature, be it phonological, lexical or otherwise.

For example, in Spain an isogloss separates the areas where in Spanish the "s" preceding a consonant is pronounced as a sibilant ("chiste", "mosca") of those in which aspirated ("chihte", "mohca"); another isogloss separates areas where words are distinguished by the s/z opposition (such as "dipple" and "caso") from areas where they are not; and so on. But isoglosses are not necessarily recognized in administrative divisions. River Plate Spanish, for example, is defined by various isoglosses that group parts of Argentina and Uruguay, however leaving out part of the territory of these countries. Also the Speyer line and the Benrath line are notorious isoglosses within the West Germanic languages.

Isogloss bundles

The Massa-Senigallia line is a beam of isogloss that separate the Galloitálic languages from the central Italian language.

When several isoglosses have a close path, we speak of "isogloss bundles". Isogloss bundles are often concentrated in transition zones between related linguistic varieties. The Massa-Senigallia line in Italy is an example of an isogloss bundle.

It is not uncommon to see loose use of the term "isogloss" to designate the lines that separate languages and dialects. Usually two related languages share a certain number of features but there are isoglosses that separate them and allow them to be distinguished systematically. For that reason, the boundaries between different languages are occasionally spoken of as isoglosses or bundles of isoglosses. However, from a strict linguistic point of view, it is not possible to define exactly whether two varieties constitute different languages or even different dialects, since there is often a continuous gradation, especially when there is a geolectal continuum or a set of mutually intelligible varieties. Since intelligibility is frequently a gradual but not a discrete phenomenon, between geographical areas, it may be impossible from the linguistic point of view to separate the varieties as languages, so the decision may be based on political and cultural affinities. Only between unrelated or very distantly related languages is it easy to distinguish geographically linguistic boundaries.

Even so, it is often marked as "main isogloss" the one that coincides with the greatest number of isoglosses and is marked as a linguistic or dialectal isogloss.