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1. In DIALECT geography, an area within which a feature is used predominantly or exclusively. Such a feature (phonological, morphological, syntactic, semantic, lexical, or other) usually contrasts with some similar feature in adjoining areas. Thus, some native speakers of English pronounce /r/ after a vowel, as in barn, hard, car, while others do not: in the US this postvocalic /r/ is normally present in the Chicago area but absent in the Boston area. Such distinct areas are isoglosses.

2. More commonly, the line on a dialect map which bounds the area of a certain usage. In England, an isogloss that stretches from the mouth of the Severn to Portsmouth separates the area of initial spoken /v/ from that of /f/, as in vinger/finger, Vriday/Friday, the v-forms being south-west of the line. No two isoglosses coincide exactly; there is always a transition area of partial overlapping. See DIALECTOLOGY.