isogloss

An isogloss is a geographic boundary that marks the distribution of a linguistic feature, separating regions where the feature is used from those where it is not. In dialect geography, isoglosses are lines on maps that indicate whether a phonological, lexical, or syntactic variant occurs in a given area. The term, from Greek isos “equal” and glossa “tongue,” was introduced by the German linguist Johannes Schmidt in the late 19th century.

On a dialect map, an isogloss represents a boundary for a specific feature. It may denote a particular pronunciation, a certain word, or a grammatical construction. Isoglosses can intersect and form networks, and when several lines cluster, they may delineate broader dialect zones or reflect a dialect continuum in which variation changes gradually across space rather than abruptly.

Isoglosses are descriptive tools used to study how language varies regionally, how features spread through contact, and how historical factors have shaped linguistic geography. They are based on observed data and are subject to sampling limitations; thus, boundaries are often approximate and may appear fuzzy rather than sharp. Social factors such as age, education, mobility, and urbanization can influence feature usage, meaning that isoglosses capture current distributions rather than fixed, timeless borders.

In practice, the concept can extend to multiple features at once, sometimes described as thick isoglosses when several features indicate a common boundary, underscoring that language change often involves more than a single feature.