The cot–caught merger is a sound change present in some dialects of English where speakers do not distinguish the vowels in "cot" and "caught". Names like "cot–caught merger" and lot–thought merger come from the minimal pairs that are lost as a result of this sound change. The phonemes involved in the cot-caught merger are represented in the International Phonetic Alphabet as /ɒ/ and /ɔ/, respectively. These vowels are both low and back—as can be seen in the IPA chart—and is sometimes referred to as the low back merger. The father-bother merger that spread through North America in the eighteenth and nineteenth centuries has resulted in many dialects having no vowel difference in words like "father", "lot", and "thought".
Vowels beside dots are: unrounded • rounded
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:
|/ɑ/ or /ɒ/ (written a, o, ol)||/ɔ/ (written au, aw, al, ough)||IPA (using ⟨ɒ⟩ for the merged vowel)||Notes|
|modeling||maudlin||ˈmɒdlɪn||With G-dropping. Also /ˈmɒdəlɪn/.|
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're stressed.
Speakers with the merger in northeastern New England still maintain a phonemic distinction between a fronted and unrounded /ɑ/ (phonetically [ä]) and a back and usually rounded /ɔ/ (phonetically [ɒ]), 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 [kɒt], they pronounce cart as [kät].
Labov et al. also reveal that, for about 15% of respondents, a specific /ɑ/–/ɔ/ merger before /n/ but not before /t/ (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.
According to Labov, Ash, and Boberg, the merger in North America is most strongly resisted in three regions:
In these areas, sociolinguists have studied three techniques that speakers use to preserve the contrast. The first was fronting of /ɑ/ found in the Inland North. In this technique, speakers advance the LOT vowel /ɑ/ as far as the cardinal [a] (the open front unrounded vowel). This is also met with the raising of the TRAP vowel /æ/ to [eə] in all instances. (Which vowel triggered which other vowel is still debated.)
The second technique of resistance to the merger is the raising of the THOUGHT vowel /ɔ/ found in the New York City and the mid-Atlantic region's accents. In areas that don't use this technique, sometimes /ɔ/ is pronounced closer to [ɔ̝]. On the contrary, in this technique, either /ɔ/ retains its historical high (raised) value [ɔ], or it is raised even higher to [ɔə⁓oə], or (in the extreme case) even [ʊə].
The third technique is found in the South. This is the result of vowel breaking in Southern American English, where /ɔ/ is broken to [ɒʊ], keeping it distinct from the LOT vowel. Many Southerners, however, are beginning to embrace the merger, particularly Southerners who are younger or urban.
African-American Vernacular English accents have traditionally resisted the cot-caught merger, with LOT pronounced [ä] and THOUGHT traditionally pronounced [ɒɔ], though now often [ɒ~ɔə]. Early 2000s research has shown that this resistance may continue to be reinforced by the fronting of LOT, linked through a chain shift of vowels to the raising of the TRAP, DRESS, and perhaps KIT vowels. This chain shift is called the "African American Shift". However there is evidence of AAVE speakers picking up the cot-caught merger in Pittsburgh and Charleston, South Carolina.
Theories of the origin of the merger exist, with two competing scenarios. One group of scholars argues for an independent North American development, while others argue for contact-induced language change via Scottish immigrants (e.g. Dollinger 2010), in which a role is afforded to Canadian English, where the spread from East to West was completed more quickly than in the US. Others consider the issue unresolved (Boberg 2010: 199?).
In London's Cockney accent, a cot–caught merger is possible only in rapid speech. The THOUGHT vowel has two phonemically distinct variants: closer /oː/ (phonetically [oː ~ oʊ ~ ɔo]) and more open /ɔə/ (phonetically [ɔə ~ ɔwə ~ ɔː]). The more open variant is sometimes neutralized in rapid speech with the LOT vowel /ɒ/ (phonetically [ɒ ~ ɔ]) in utterances such as [sˈfɔðɛn] (phonemically /ɑɪ wəz ˈfɔə ðen/) for I was four then. Otherwise /ɔə/ is still readily distinguished from /ɒ/ by length.
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 /a/ and /ɔ/.