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 that distinguishes High German from the other West Germanic languages and the La Spezia–Rimini Line that 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 cognates with the /y/-containing French words.

One of the most well-known isoglosses is the centum-satem isogloss.

Similar to an isogloss, an isograph is a distinguishing feature of a writing system. Both concepts are also used in historical linguistics.

The centum-satem isogloss of the Indo-European language family relates to the different evolution of the dorsal consonants of Proto-Indo-European (PIE). In the standard reconstruction, three series of dorsals are recognised:

In some branches (for example Greek, Italic and Germanic), the palatals merged with the velars: PIE *keup- "tremble (inwardly)" became Latin cupiō "desire" and *ḱm̥tom "hundred" became Latin centum (pronounced [kentum]); but *kʷo- "interrogative pronoun" became quō "how? where?". They are known as centum branches, named after the Latin word for hundred.

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