More recently he has studied ongoing changes in the phonology of English as spoken in the United States, and the origins and patterns of chain shifts of vowels (one sound replacing a second, replacing a third, in a complete chain). In the "Atlas of North American English" (2006), he and his co-authors find three major divergent chain shifts taking place today: a Southern Shift (in Appalachia and southern coastal regions), a Northern Cities Vowel Shift affecting a region from Madison, Wisconsin, east to Utica, New York, and a Canadian Shift affecting most of Canada, as well as some areas in the Western and Midwestern (Midland) United States, in addition to several minor chain shifts in smaller regions.