as the Don–dawn merger. Resistance According to Labov, Ash, and Boberg, the merger in North America is most strongly resisted in three regions: The "South", somewhat excluding Texas and Florida. The "Inland North", encompassing the eastern and central Great Lakes region (on the U.S. side of the border) The "Northeast Corridor" along the Atlantic coast, ranging from Baltimore to Philadelphia to New York City to Providence. However, the merger is common in Boston and further northern New England. In the three American regions above, sociolinguists have studied three phonetic shifts that can explain their resistance to the merger. The first is the fronting of found in the Inland North; speakers advance the vowel as far as the cardinal (the open front unrounded vowel), thus allowing the vowel to lower into the phonetic environment of without any merger taking place. The second situation is the raising of the vowel found in the New York City, Philadelphia and Baltimore accents, in which the vowel is raised and diphthongized to , or, less commonly, , thus keeping that vowel notably distinct from the vowel . The third situation occurs in the South, in which vowel breaking results in being pronounced as upgliding , keeping it distinct from . None of these three phonetic shifts, however, is certain to preserve the contrast for all speakers in these regions. Some speakers in all three regions, particularly younger ones, are beginning to exhibit the merger despite the fact that each region's phonetics should theoretically block it. African American Vernacular English accents have traditionally resisted the cot–caught merger, with pronounced and traditionally pronounced , though now often . Early 2000s research has shown that this resistance may continue to be reinforced by the fronting of , linked through a chain shift of vowels to the raising of the , , and perhaps vowels. This chain shift is called the "African American Shift". However, there is still evidence of AAVE speakers picking up the cot–caught merger in Pittsburgh, Pennsylvania, in Charleston, South Carolina, Florida and Georgia, and in parts of California. Origin In North America, the first evidence of the merger (or its initial conditions) comes from western Pennsylvania as far back as the data show. From there, it entered Upper Canada (what is now Ontario). In the mid-19th century, the merger also independently began in eastern New England, possibly influencing the Canadian Maritimes, though the merger is