Phonological history of English low back vowels
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The phonology of the low back vowels of the English language has undergone changes both overall and with regional variations, dating from Late Middle English (c. 1400) to the present. The sound changes heard in modern English mostly begin with the Great Vowel Shift, and continue through the development and recognition of the General American dialect and the cot–caught merger.
Late Middle English 
In Late Middle English (c. 1400) the following low back vowels were present, distinguished by length:
- /ɔ/ as in dog
- /ɔː/ as in boat
16th century changes 
By 1600 the following changes had occurred:
- The long vowel /ɔː/ of boat had been raised to /oː/ as a result of the Great Vowel Shift.
- The diphthong /ɑʊ/ found in words such as cause, law, all, salt, psalm, half, change, chamber, dance had become an open back diphthong /ɔ(ː)ʊ/.
- The diphthong /ɔʊ/ found in low and soul had become /oʊ/.
- Before nonprevocalic /r/, short /ɔ/ had become lowered to /ɒ/, thus corn, /kɒrn/.
There were thus three low back monophthongs at this time: /ɔ/ as in dog, /ɔː/ as in low and (before /r/), in more and /ɒ/ in corn.
17th century changes 
By 1700 the following further developments had taken place:
- The monophthong /ɔː/ of soul was raised to /oː/, merging with boat (see tow–toe merger). This change did not happen before /r/.
- Short /a/ merged with /ɔ/ when following a /w/, as in want, quality. The merger was suppressed before a velar consonant, as in quack, twang, wag, wax. Before nonprevocalic /r/, the vowel was opened and lengthened, merging instead with /ɒː/, as in war.
- Short /ɔ/ had begun to partake in lengthening before a nonprevocalic voiceless fricative. This resulted in words like broth, cost, and off having /ɒː/ instead of /ɔ/.
- Short /ɒ/ before /r/ lengthened to /ɒː/: thus corn, /kɒːrn/
- In words such as change and chamber, the pronunciation /ɒː/ was gradually replaced in the standard language by a variant with /eː/, derived from Middle English /aː/. This explains the contemporary pronunciation of these words with /eɪ/.
This left the language with three low back vowels:
- /ɔ/ in dog and want.
- /ɔː/ in more.
- /ɒː/ in cause, and cost, and corn.
Father–bother merger 
The father–bother merger is a merger of the Early Modern English vowels /ɑː/ and /ɒ/ that occurs in almost all varieties of North American English (exceptions are accents in northeastern New England, such as the Boston accent, and in New York City). In those accents with the merger father and bother rhyme, Kahn and con are homophonous as [kɑn], and Saab and sob are homophonous as [sɑb]. Balm and bomb may also be homophones as /bɑm/: however this merger is prevented for some speakers by the reintroduction of the historical l into balm. Another possible merger is lager and logger, for some but by no means all speakers, particularly those with the lot–cloth split when it extends to -og words including log. Unrounding of EME /ɒ/ is found also in Norwich, the West Country, the West Midlands and in Hiberno-English, but apparently with no phonemic merger (typically because vowel length remains phonemic). The card–cord merger is a roughly similar merger, but occurring before r.
Lot–cloth split 
The lot–cloth split is the result of a late 17th century sound change that lengthened /ɒ/ to [ɒː] before voiceless fricatives, and also before /n/ in the word gone. In some accents, the lengthened [ɒː] was raised, merging with the /ɔː/ of words like thought. The sound change is most consistent in the last syllable of a word, and much less so elsewhere (see below). Some words that entered the language later, especially when used more in writing than speech, are exempt from the lengthening, e.g. joss and Goth with the short vowel.
The cot-caught merger, discussed below, has removed the distinction in some dialects, and may make the lot-cloth split less noticeable.
As a result of the lengthening and raising, in the above-mentioned accents cross rhymes with sauce, and soft and cloth also have the vowel /ɔː/. Accents affected by this change include American English and, originally, RP, although today words of this group almost always have short /ɒ/ in RP. The split still exists in some older RP speakers, including Queen Elizabeth II.
The lengthening and raising generally happened before the fricatives /f/, /θ/ and /s/. In American English the raising was extended to the environment before /ŋ/ and /ɡ/, and in a few words before /k/ as well, giving pronunciations like /lɔŋ/ for long, /dɔɡ/ for dog and /tʃɔklɨt/ for chocolate. (Obviously, in accents of American English that are subject to the cot–caught merger, there is no difference between words that did and those that did not undergo the change.)
In the varieties of American English that have the lot–cloth split, the lot vowel is usually symbolized as /ɑ/ and the cloth vowel as /ɔ/, since the distinction is usually one of vowel quality rather than length. The actual pronunciation of these vowels may vary somewhat from the symbol used to denote them; e.g. /ɔ/ is often pronounced closer to a low back rounded vowel [ɒ], and /ɑ/ is sometimes fronted to a low central vowel [ä]. Some words vary as to which vowel they have. For example, words that end in -og like frog, hog, fog, log, bog etc. have /ɑ/ rather than /ɔ/ in some accents.
There are also significant complexities in the pronunciation of written o occurring before one of the triggering phonemes /f θ s ŋ ɡ/ in a non-final syllable. Normally, when a word is formed by adding a suffix to an existing word, the vowel quality is maintained. Hence /ɔ/ occurs in crossing, crosser, crosses because it occurs in cross; likewise in longing, longer, longest because it occurs in long. In words not formed this way, however, the phoneme /ɑ/ tends to occur even before a triggering phoneme. For example, possible, jostle, impostor, profit, Gothic, bongo, Congo, and boggle all have /ɑ/. But there are numerous exceptions (e.g. Boston with /ɔ/), including across apparent rhyming pairs (e.g. roster with /ɑ/ but foster often with /ɔ/), as well as a good deal of variation (coffee, offer, donkey, soggy, boondoggle, etc. with either /ɑ/ or /ɔ/, depending on the speaker).
The word on is pronounced with /ɑ/ in the North, but with /ɔ/ in the Midland, Mid-Atlantic and South.
Cot–caught merger 
The cot–caught merger (also known as the low back merger) is a phonemic merger, a sound change, that occurs in some varieties of English. The merger occurs in some accents of Scottish English and to some extent in Mid Ulster English but is best known as a phenomenon of many varieties of North American English. Many speakers of Singaporean English also merge the vowels of "cot" and "caught".
The sound change causes the vowel in caught, talk, and small to be pronounced like the vowel in cot, rock, and doll, so that cot and caught, for example, become homophones, and the two vowels merge into a single phoneme. The change does not affect a vowel followed by /r/, so barn and born remain distinct, and starring and warring do not rhyme.
The presence of the merger and its absence are both found in many different regions of the continent, 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.
According to Labov, Ash, and Boberg, the merger does not generally occur in the southern United States (with exceptions), along most of the American side of the Great Lakes region, or in the "Northeast Corridor" extended metropolitan region from Providence, Rhode Island to Baltimore. Areas where it occurs include:
- Northeastern New England
- the Pittsburgh area (see Pittsburghese)
- The Western United States
- Due to an apparent spread of the merger towards the center of the United States (from both the western and eastern states), portions of the Midwest also feature the merger:
The distribution of the merger is complex, even without taking into account the mobility of the American population; 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), younger speakers in Kansas, Nebraska, and the Dakotas exhibit the merger while speakers older than 40 typically do 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 approximately 60% of American English speakers preserve the contrast, while approximately 40% make the merger, although in a more recent interview, Labov stated that "Half of this country has a merger of the word classes, cot, caught, don, dawn, hock, hawk."
For merged speakers in Canada and most of the United States, the two sounds [ɑ] and [ɔ] are allophones; they often do not perceive differences in their usage, hear neither of them as a separate phoneme, and hear the distinct vowels used by speakers whose dialects do distinguish them as variations on the same vowel. They hear the broad A of British Received Pronunciation as the same, single vowel sound. But in Received Pronunciation, there are three sounds distinguished: the long /ɑː/ of cart, the long /ɔː/ of caught, and the short rounded /ɒ/ of cot.
Speakers with the merger in northeastern New England still maintain a phonemic distinction between a fronted and unrounded /aː/ and a back and usually rounded /ɒː/, 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 [kɒːt], they pronounce cart as [kaːt].
Labov et al. also reveal that about 15% of respondents have the merger before /n/ but not before /t/, so that Don and Dawn are homophonous, but cot and caught are not. A much smaller group (about 4%) has the reverse situation: cot and caught are homophonous but Don and Dawn are distinct.
Speakers who have the "father–bother" merger in addition to the "cot–caught" merger may have further homophones such as ah/awe, Pa/paw, Pa's/pause-paws and shah–Shaw.
|A - lengthening||aː|
|Loss of distinctive length||ɔ||ɒ||(ɑ)||ɑ|
|General American Output||ɔ||ɔ||ɑ||ɑ|
See also 
- Barber, pp. 108,111
- Barber, pp. 108, 111, 116
- Barber, pp. 121-122
- Barber, p. 123
- Barber, p. 108
- Merriam Webster Pronunciation Guide
- Wells, John C. (1982). Accents of English. Cambridge: Cambridge University Press. ISBN 0-521-22919-7 (vol. 1), ISBN 0-521-24224-X (vol. 2), ISBN 0-521-24225-8 (vol. 3) Check
|isbn=value (help)., pp. 136–37, 203–6, 234, 245–47, 339–40, 400, 419, 443, 576
- Labov et al. (2006), p. 171
- Labov et al. (2006), p 169
- According to the online edition of the Oxford Advanced American Dictionary, .
- Gordon (2005)
- Gordon (2005), citing the TELSUR project
- The 2003 Harvard Dialect Survey
- Barber, Charles Laurence (1997). Early modern English (second ed.). Edinburgh: Edinburgh University Press. ISBN 0-7486-0835-4.
- Gordon, Matthew J. (2005), "The Midwest Accent", American Varieties (PBS), retrieved August 29, 2010
- Labov, William; Aash, Sharon; Boberg, Charles (2006). The Atlas of North American English: Phonetics, Phonology, and Sound Change: a Multimedia Reference Tool. Berlin ; New York: Walter de Gruyter. ISBN 3-11-016746-8.
- 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 /n/ and /t/
- Chapter 13 of the Atlas of North American English, which discusses the "short-o" configuration of various American accents