Cot–caught merger

The cot–caught merger, also known as the  merger or low back merger, is a sound change present in some dialects of English where speakers do not distinguish the vowel phonemes in words like cot versus caught. Cot and caught (along with bot and bought, pond and pawned, etc.) is an example of a minimal pair that is lost as a result of this sound change. The phonemes involved in the cot–caught merger, the low back vowels, are typically represented in the International Phonetic Alphabet as and  or, in North America, as  and  (except in regions that do not have the father–bother merger). The merger is typical of most Canadian and Scottish English dialects as well as some Irish and U.S. English dialects.

An additional vowel merger, the father–bother merger, which spread through North America in the eighteenth and nineteenth centuries, has resulted today in a three-way merger in which most Canadian and many U.S. accents have no vowel difference in words like,  , and. However, as in  participates in a separate phenomenon in most North American English dialects: the  merger, in which this vowel before  can be phonemicized as the  vowel, transcribed together variously thus as  or.

Overview
The shift causes the vowel sound in words like cot, nod and stock and the vowel sound in words like caught, gnawed and stalk to merge into a single phoneme; therefore the pairs cot and caught, stock and stalk, nod and gnawed become perfect homophones, and shock and talk, for example, become perfect rhymes. The cot–caught merger is completed in the following dialects:
 * Some English of the British Isles, outside of England:
 * Most Scottish English, towards
 * Broad and traditional Irish English
 * Some northern Ulster English including in conservative mid Ulster English towards and in Ulster Scots English towards
 * Much of the English of North America:
 * Certain varieties of American English, including:
 * Pittsburgh English, towards (with the father–bother merger)
 * Much of New England English towards (in Boston, particularly towards ), and Northern New England generally, but traditionally not Southern New England
 * Western American English (with the father–bother merger) towards
 * Cajun English, Upper Midwestern English, Chicano English, and some Western New England English (with the father–bother merger) towards
 * Nearly all Canadian English, including:
 * Standard Canadian English towards (with the father–bother merger)
 * Maritimer and Newfoundland English, towards (with the father–bother merger)
 * Much Indian English towards or
 * Some Singaporean English

North American English
Nowhere is the shift more complex than in North American English. The presence of the merger and its absence are both found in many different regions of the North American continent, where it has been studied in greatest depth, and in both urban and rural environments. The symbols traditionally used to transcribe the vowels in the words cot and caught as spoken in American English are ⟨ɑ⟩ and ⟨ɔ⟩, respectively, although their precise phonetic values may vary, as does the phonetic value of the merged vowel in the regions where the merger occurs.

Even without taking into account the mobility of the American population, the distribution of the merger is still complex; there are pockets of speakers with the merger in areas that lack it, and vice versa. There are areas where the merger has only partially occurred, or is in a state of transition. For example, based on research directed by William Labov (using telephone surveys) in the 1990s, younger speakers in Kansas, Nebraska, and the Dakotas exhibited the merger while speakers older than 40 typically did not. The 2003 Harvard Dialect Survey, in which subjects did not necessarily grow up in the place they identified as the source of their dialect features, indicates that there are speakers of both merging and contrast-preserving accents throughout the country, though the basic isoglosses are almost identical to those revealed by Labov's 1996 telephone survey. Both surveys indicate that, as of the 1990s, approximately 60% of American English speakers preserved the contrast, while approximately 40% merged the phonemes. Further complicating matters are speakers who merge the phonemes in some contexts but not others, or merge them when the words are spoken unstressed or casually but not when they are stressed.

Speakers with the merger in northeastern New England still maintain a phonemic distinction between a fronted and unrounded (phonetically ) and a back and usually rounded  (phonetically ), because in northeastern New England (unlike in Canada and the Western United States), the cot–caught merger occurred without the father–bother merger. Thus, although northeastern New Englanders pronounce both cot and caught as, they pronounce cart as.

Labov et al. also reveal that, for about 15% of respondents, a specific – merger before but not before  (or other consonants) is in effect, so that Don and dawn are homophonous, but cot and caught are not. In this case, a distinct vowel shift (which overlaps with the cot–caught merger for all speakers who have indeed completed the cot–caught merger) is taking place, identified 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 shows. 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 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, 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.

Scotland
Outside North America, another dialect featuring the merger is Scottish English, where the merged vowel has a quality around [ɔ̞]. Like in New England English, the cot–caught merger occurred without the father–bother merger. Therefore, speakers still retain the distinction between in  and  in.

India
The merger is also quite prevalent in Indian English, possibly due to contact with Scottish English. In particular, the vowel may be lengthened to merge with the  vowel. However, there are also speakers who maintain a distinction in length and/or quality. Like in Scottish English, this vowel is not usually merged with  in General Indian English.