Isogloss

An isogloss, also called a heterogloss (see Etymology below), is the geographic boundary of a certain linguistic feature, such as the pronunciation of a vowel, the meaning of a word, or the use of some syntactic feature. Major dialects are typically demarcated by groups of isoglosses such as the Benrath line distinguishes High German from the other West Germanic languages; and the La Spezia–Rimini Line divides the Northern Italian dialects from Central Italian dialects. However, an individual isogloss may or may not have any coincidence with a language border. For example, the front-rounding of /y/ cuts across France and Germany, while the /y/ is absent from Italian and Spanish words that are cognate with the /y/-containing French words.