The cot-caught merger (also known as the low back merger) is a phonemic merger, a sound change, that occurs in some varieties of English...is best known as a phenomenon of many varieties of North American English.
It is very widespread across Canada, the Boston, Massachusetts area (see Boston accent) and northeastern New England, the Pittsburgh area (see Pittsburghese), and is also heard throughout the western U.S. The latter seems to be the source of its introduction into the Midwest as it appears to be spreading eastward. A recent survey directed by William Labov of the University of Pennsylvania has shown that the merger can be found today among younger generations (roughly people under 40) in Kansas, Nebraska, and the Dakotas