Northern Cities Shift

A recent sound change affecting many of the vowels in the north-eastern accents of American English. It is most prominent in the cities on the United States side of the Great Lakes, such as Chicago, Buffalo, and Cleveland, and is happening to a lesser extent further south. It does not apply to Canadian English, nor to the New York or Boston accents.

A chain has occurred (or is occurring), where one vowel moves into the space occupied by another, which meanwhile moves to a different space. This is called a drag-chain if it's initiated by the leading vowel and a push-chain if by the last vowel. I don't know whether it's known how the Northern Cities Shift was caused.

The part of the north-west affected by the shift still has different vowels in cot and caught (and in Don/Dawn and stock/stalk), whereas much of North America, including Canada and parts of New England and Pennsylvania, has undergone the cot/caught merger. Both these vowels have taken part in the shift, which produces these effects:

                       sounds like
Northern Cities      General American     SAMPA/IPA

    stuck               stalk               O
    stalk               stock               A
    stock               stack               a
    stack              (steack)             eæ

That is, the N.C. word stuck is pronounced much like the way stalk is by those western or mid-western Americans who still keep it distinct from stock. The N.C. stalk will sound the same as stalk as said by a General American speaker who has merged stock/stalk, but the N.C. hasn't merged them: the back ah-vowel of stock moves forward to something approaching stack. Finally, their stack has moved even higher and acquired a diphthong character.

The Northern Cities Shift was first documented by William Labov et al. in 1973, so it's very recent for something that makes such a big difference to regional accents. Other historically significant vowel system rearrangements in English include the Great Vowel Shift of the 1400s and the diphthong shift of the 1800s that affected Cockney and Australian.

www.ic.arizona.edu/~lsp/Northeast/ncshift/ncshift.html

Labov, William, Malcah Yaeger, and Richard Steiner. 1973. The Quantitative study of sound change in progress. Philadelphia: U.S. Regional Survey.