in evidence as early as the 1830s in both regions of Canada: Ontario and the Maritimes. Fifty years later, the merger "was already more established in Canada" than in its two U.S. places of origin. In Canadian English, further westward spread was completed more quickly than in English of the United States. Two traditional theories of the merger's origins have been longstanding in linguistics: one group of scholars argues for an independent North American development, while others argue for contact-induced language change via Scots-Irish or Scottish immigrants to North America. In fact, both theories may be true but for different regions. The merger's appearance in western Pennsylvania is better explained as an effect of Scots-Irish settlement,<ref>Evanini, Keelan (2009). "The permeability of dialect boundaries: A case study of the region surrounding Erie, Pennsylvania". University of Pennsylvania; dissertations available from ProQuest. AAI3405374. pp. 254-255.</ref> but in eastern New England, and perhaps the American West, as an internal structural development. Canadian linguist Charles Boberg considers the issue unresolved. A third theory has been used to explain the merger's appearance specifically in northeastern Pennsylvania: an influx of Polish- and other Slavic-language speakers whose learner English failed to maintain the distinction. England In London's Cockney accent, a cot–caught merger is possible only in rapid speech. The vowel has two phonemically distinct variants: closer (phonetically ) and more open (phonetically ). The more open variant is sometimes neutralized in rapid speech with the vowel (phonetically ) in utterances such as (phonemically ) for I was four then''. Otherwise is still readily distinguished from by length. Scotland Outside North America, another dialect featuring the merger is Scottish English. Like in New England English, the cot–caught merger occurred without the father–bother merger. Therefore, speakers still retain the distinction between and . See also Phonological history of English open back vowels Notes References Bibliography External links Map of the cot–caught merger from the 2003 Harvard Dialect Survey Map of the cot–caught merger from Labov's 1996 telephone survey Description of the cot–caught merger in the Phonological Atlas Map of the cot–caught merger before and Chapter 13 of the Atlas of North American English, which discusses the "short-o" configuration of various American accents Dialects of English Splits and mergers in English phonology