A variant of a language in terms of vocabulary grammar, syntax, and pronunciation. Dialect differences arise and persist because of geographical barriers (the Ohio River, for instance, helps to define the division between the dialect areas of the North and the South USA), political boundaries, settlement patterns, migration and immigration routes, territorial conquest, and language contact (Rickford (2002) Discover). See also Langevelde and Pellenbar (2001) Tijdschrift 92, 3. Dialectology is the study of social and linguistic variations within a language; dialect geography is the study of local differentiations in a speech area: see J. K. Chambers and P. Trudgill (1998). Dialectometric techniques analyse linguistic variation quantitatively, to map geographic patterns of individual linguistic variants, such as which word is used for a particular concept in a language area, or which sounds are used in particular words (Kretzschmar (2006) Literary & Ling. Comp. 21, 4).