Isogloss

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High German subdivides into Upper German (green) and Central German (blue), and is distinguished from Low Franconian and Low German (yellow). The main isoglosses, the Benrath and Speyer lines are marked in black.

An isogloss is the geographical boundary or delineation of a certain linguistic feature, e.g. the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature. Major dialects are typically demarcated by whole bundles of isoglosses, e.g. the Benrath line that distinguishes High German from the other West Germanic languages; or the La Spezia-Rimini Line which divides the eastern Romance languages from the western ones. Undoubtedly, the largest well-known isogloss is the Centum-Satem isogloss, which traditionally separates the Indo-European languages into two distinct categories.

A major isogloss in American English has been identified as the North-Midland isogloss, which demarcates numerous linguistic features, including the Northern Cities vowel shift: regions north of the line (including western New York; Cleveland, Ohio; lower Michigan; northern Illinois; and eastern Wisconsin) are subject to the shift and regions south of the line (including Pennsylvania, central and southern Ohio, and most of Indiana) are not.

The name is inspired by contour lines or isopleths such as isobar, etc.; however, the isogloss separates rather than connects points of equal language (perhaps one could say it connects points of indefinite language).