Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
CHANGE AND VARIATION IN LANGUAGE
Most linguists distinguish between linguistic change and linguistic variation as follows: Linguistic change occurs over time; for example, the differences in spelling and pronunciation between Middle English niht and Modern English night represent linguistic changes that developed between (roughly) the fourteenth and the sixteenth or seventeenth centuries. In contrast, linguistic variation exists at one given time. One variety is represented by the different pronunciations of a word like barn by an Eastern New Englander and by a speaker of the Great Lakes Northern dialect; another is illustrated in the difference between the chief British and American meanings of the noun vest.
