The phonology of theopenback vowels of theEnglish language has undergone changes both overall and with regional variations, throughOld andMiddle English to the present. The sounds heard in modern English were significantly influenced by theGreat Vowel Shift, as well as more recent developments in some dialects such as thecot–caught merger.

In theOld English vowel system, the vowels in the open back area were unrounded:/ɑ/,/ɑː/. There were also rounded back vowels of mid-height:/o/,/oː/. Thecorresponding spellings were⟨a⟩ and⟨o⟩, with the length distinctions not normally marked; in modern editions of Old English texts, the long vowels are often written⟨ā⟩,⟨ō⟩.

As the Old English (OE) system developed into that ofMiddle English (ME), the OE short vowel/ɑ/ merged with the fronted/æ/ to become a more central ME/a/. Meanwhile, the OE long vowel/ɑː/ was rounded and raised to ME/ɔː/. OE short/o/ remained relatively unchanged, becoming a short ME vowel regarded as/o/or/ɔ/, while OE long/oː/ became ME/oː/ (a higher vowel than/ɔː/). Alternative developments were also possible; seeEnglish historical vowel correspondences for details.

Later, MEopen syllable lengthening caused the short vowel/o/ to be normally changed to/ɔː/ inopen syllables. Remaining instances of the short vowel/o/ also tended to become lower. Hence in Late Middle English (around 1400) the following open back vowels were present, distinguished by length:^[1]

• /ɔ/, spelt⟨o⟩, as indog,god
• /ɔː/, often spelt⟨oa⟩, or⟨o⟩ before consonant+vowel or certain consonant pairs, as inboat,whole,old

By 1600, the following changes had occurred:

• The long vowel/ɔː/ ofboat had been raised to/oː/ as a result of theGreat Vowel Shift.
• The diphthong/aw/ found in words such ascause,law,all,salt,psalm,half,change,chamber,dance had become an open back monophthong/ɔː/ or/ɑː/.
• At this time, the short/ɔ/ indog was lowered to/ɒ/

There were thus two open back monophthongs:

• /ɒ/ as inlot
• /ɔː/ or/ɑː/ as incause

and one open back diphthong:

• /ɔw/ as inlow

By 1700, the following further developments had taken place:

• The diphthong/ɔw/ ofsoul was raised to/ow/, and then monophthongized to/oː/, merging withboat (seetoe–tow merger). Before/r/, this vowel further merged with/ɔː/ due to the horse–hoarse merger except in some varieties, as currently seen inIrish English,Scottish English andAfrican American Vernacular English.
• Short/wa/ was retracted and rounded to/wɒ/. The shift was suppressed before avelar consonant, as inquack,twang,wag,wax, and also was suppressed by analogy inswam (the ("strong-verb") past tense ofswim). The change of/wa/ to/wɒ/ did not occur inMid-Ulster English.
• /ɒ/ had begun to partake in lengthening and raising before a nonprevocalic voiceless fricative or /r/. That resulted in words likebroth,cost andoff having/ɒː/ instead of/ɒ/, and was the start of theLOT–CLOTH split (see further below).
• In words such aschange andchamber, the pronunciation/ɔː/ was gradually replaced in the standard language by a variant with/eː/, derived from Middle English/aː/. That explains the contemporary pronunciation of these words with/eɪ/.
• However, when/ɔː/ preceded/f/, as inlaugh andhalf,/ɔː/ was shifted to/æ/ instead, derived from Middle English/a/.
• Anunrounded back vowel/ɑː/ developed, found in certain classes of words that had previously had/a/, likestart,father andpalm.

That left the standard form of the language with four open back vowels:

• /ɒ/ inlot andwant.
• /ɒː/ incloth andcost.
• /ɑː/ instart,father andpalm.
• /ɔː/ intor,cause, andcorn.

From the 18th century on, the following changes have occurred:

• The three-way distinction between/ɒ/,/ɒː/, and/ɔː/ was simplified in one of two ways:
  • InGeneral American and old-fashioned RP,/ɒː/ was raised to/ɔː/, merging with the vowel inTHOUGHT (thecloth-thought merger).
  • In many accents of England, the lengthening of theCLOTH set was undone, restoring the short pronunciation/ɒ/. This became standard RP by the mid-20th century.
• In General American, thelot vowel has become unrounded and merged into/ɑ/ (thefather–bother merger).

This leaves RP with three back vowels:

• /ɒ/ inlot,want,cloth, andcost.
• /ɔː/ intor,cause, andcorn.
• /ɑː/ instart,father, andpalm.

and General American with two:

• /ɑ/ inlot,want,start,father, andpalm.
• /ɔ/ intor,cause,corn,cloth andcost.

In a few varieties of English, the vowel inlot is unrounded, pronounced toward [ɑ]. This is found in the following dialects:

• Most ofIrish English
• Much of the Caribbean
• Norwich
• TheWest Country and theWest Midlands of England
• Most ofNorth American English
  • Excluding northeasternNew England English andWestern Pennsylvania English, where it is typically raised toward/ɔ/, merging with the vowel inthought.

There's also evidence for it inSouth East England as early as the late 16th century and as late as the 19th century.^[2][3]

Linguists^[which?] disagree as to whether the unrounding of thelot vowel occurred independently in North America (probably occurring around the end of the 17th century) or was imported from certain types of speech current in Britain at that time.^{[citation needed]}

In such accents outside of North America,lot typically is pronounced as[lɑt],^[4] therefore being kept distinct from the vowel inpalm, pronounced[pɑːm] or[paːm]. However, the major exception to this isNorth American English, where the vowel is lengthened to merge with the vowel inpalm, as described below. This merger is called theLOT–PALM merger or more commonly thefather–bother merger. (See further below.)

Thefather–bother merger is a phonemic merger of the lexical setsLOT andPALM. It represents unroundedlot, as detailed above, taken a step further. On top of being unrounded, the length distinction between the vowel inlot andbother and the vowel inpalm andfather is lost, so that the two groups merge. This causesfather andbother to become rhymes.

This occurs in the great majority of North American accents; of the North American dialects that have unroundedlot, the only notable exception to the merger isNew York City English, where the opposition with the[ɑ]-type vowel is somewhat tenuous.^[5][6]

Examples of possiblehomophones resulting from the merger includeKhan andcon (/kɑn/) as well asSaab andsob (/sɑb/).^[7]

While the accents in northeasternNew England, such as theBoston accent, also remain unmerged,lot remains rounded and merges instead withcloth andthought.^[5][6]

TheLOT–CLOTH split is the result of a late 17th-century sound change that lengthened/ɒ/ to[ɒː] beforevoicelessfricatives, and also before/n/ in the wordsgone and sometimeson. It was ultimately raised and merged with/ɔː/ of words likethought, although in some accents that vowel is actually open[ɒː]. This means thatCLOTH is not a separate vowel; rather, it means "eitherLOT orTHOUGHT, depending on the accent". 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 andGoth with the short vowel. Similar changes took place in words with⟨a⟩; seetrap–bath split and/æ/-tensing.

The cot–caught merger, discussed below, has removed the distinction in some dialects.

As a result of the lengthening and raising, in the above-mentioned accentscross rhymes withsauce, andsoft andcloth also have the vowel/ɔː/. Accents affected by this change includeAmerican English accents that lack thecot-caught merger and formerlyRP, although with the exception ofwater/wɔːtə(r)/, today words of this group almost always have short/ɒ/ in RP.

The lengthening and raising generally happened before the fricatives/f/,/θ/, and/s/, and in the wordwater for an unknown reason (compare thebroadening ofa infather). In American English, the raising was extended to the environment beforevelars/ŋ/ and/ɡ/, and sometimes before/k/ as well, giving pronunciations like/lɔŋ/ forlong,/dɔɡ/ fordog and/ˈtʃɔklət/ forchocolate.

In the varieties ofAmerican English that have the lot–cloth split, thelot vowel is usually symbolized as/ɑ/, often called the "short o" for historical reasons, as the corresponding RP vowel/ɒ/ is still short (and it contrasts with/ɑː/ as infather andstart). Thethought vowel is usually transcribed as/ɔ/ and it is often called the "open o". Its actual phonetic realization may be open[ɒ], whereas thelot vowel may be realized as central[ä]. Some words vary as to which vowel they have. For example, words that end in-og likefrog,hog,fog,log,bog etc. have/ɑ/ in some accents and/ɔ/ in others.

There are also significant complexities in the pronunciation of writteno occurring before one of the triggering phonemes/fθsŋɡ/ in a non-final syllable. In other cases, however, the use of the open o as opposed to the short o is largely predictable. Just like with/æ/-tensing and thetrap–bath split, there seems to be an open-syllable constraint. Namely, the change did not affect words with /ɑ/ in open syllables unless they were closely derived from words with/ɑ/ inclosed syllables. Hence/ɔ/ occurs incrossing,crosser,crosses because it occurs incross. In contrast,possible,jostle,impostor,profit,Gothic, andboggle all have/ɑ/. However, there are still exceptions in words likeBoston andfoster^[b].^{[11][12][13][14][15][16][17]} A further list of words is mentioned in the table below:

Some words may vary depending on the speaker like (coffee,offer,donkey,soggy,boondoggle, etc. with either/ɑ/ or/ɔ/).^{[citation needed]} Meanwhile, other words vary by region. For example, the wordon, which inNorthern American English dialects with thecot-caught merger is pronounced/ɑn/, rhyming withdon, but inMidland andSouthern American English without the merger is pronounced/ɔn/, rhyming withdawn. The isogloss for this difference, termed theON line, lies betweenNew York City and Philadelphia on the East Coast and runs West as far as speakers without the merger can be found.^[19] Pronunciation of the wordwant as/wɔnt/ is also strongly associated with the South.^[20]

Thecot–caught merger (also known as the low back merger or theLOT–THOUGHT merger) is aphonemic merger occurring in many accents of English, where the vowel sound in words likecot,nod, andstock (theLOT vowel), has merged with that ofcaught,gnawed, andstalk (theTHOUGHT vowel). For example, with the merger,cot andcaught become perfecthomophones.

TheGOAT–THOUGHT merger is a merger of the English vowels ofGOAT/oʊ/ andTHOUGHT/ɔː/ that has been reported inGeordie since the late 20th century, with a quality around [oː]. The merger is more common among younger female speakers.^[21]

The merger also exists among older speakers inBradford English with a quality around [ɔː], but younger speakers are more likely to resist the merger by fronting theGOAT vowel.^[21]

In some London accents of English, the vowel in words such asthought,force, andnorth, which merged earlier on in these varieties of English, undergoes a conditional split based on syllable structure: closed syllables have a higher vowel quality such as[oː] (possibly even[oʊ] in broad Cockney varieties), and open syllables have a lower vowel quality[ɔ̝ː] or a centering diphthong[ɔə].

Originally-open syllables with an inflectional suffix (such asbored) retain the lower vowel quality, creating minimal pairs such asbored[bɔəd] vs.board[boːd].^[22]

In broadGeordie, someTHOUGHT words (roughly, those spelled witha, as inwalk andtalk) have[aː] (which phonetically is the long counterpart ofTRAP/a/) instead of the standard[ɔː]. Those are the traditional dialect forms which are being replaced with the standard[ɔː].[aː] is therefore not necessarily a distinct phoneme in the vowel system of Geordie, also because it occurs as an allophone of/a/ before voiced consonants.^[23]

TheWRATH-ROTH merger is a merger of the English vowel ofWRATH/æ~ɑː/ into the vowel ofROTH/ɒ~ɔ(ː)/ that appears inReceived Pronunciation and some speakers ofNew Zealand English, occurring only when the vowel appears afterwr (historical/wr/)^{[citation needed]}.

The distribution of the vowel transcribed with ⟨ɑː⟩ in broad IPA varies greatly among dialects. It corresponds to/æ/,/ɒ/,/ɔː/ and (when not prevocalic within the same word)/ɑːr/ and even/ɔːr/ in other dialects:

• In non-rhotic dialects spoken outside of North America,/ɑː/ corresponds mostly to/ɑːr/ in General American and so is most often spelled⟨ar⟩. In dialects with thetrap–bath split (such as Received Pronunciation, New Zealand English and South African English), it also corresponds to GA/æ/, which means that it can also be spelled⟨a⟩ before voiceless fricatives. In those dialects,/ɒ/ and/ɔː/ are separate phonemes.
• In native words,/ɑː/ in most non-rhotic speech of North America corresponds to both/ɑːr/ in GA (RP/ɑː/) and/ɒ/ in RP, as those dialects feature thefather–bother merger.
• North American English features thefather–bother merger, where/ɑː/ often corresponds to/ɒ/ in RP. OnlyNew York City English andNew England English distinguish between the two, and with an unroundedLOT vowel. Modern-day New York City English also has rounded/ɒː/ for reflexes ofPALM, which ironically, came from an unrounded vowel.
• Thecot–caught merger usually occurs in addition to the father–bother merger. This applies to almost all of Canadian English and many varieties of American English. The result is usually/ɑ(ː)/, thePALM vowel, which is used forLOT as well. Some dialects will have/ɒ(ː)/ as the merged vowel, not/ɑ(ː)/; these includeStandard Canadian English orPittsburgh English.
  • Thecaught-cot mergerwithout thefather–bother merger is found inScottish English and most ofNew England English.
• In loanwords and names, theopen central unrounded vowel[ä] in a source language is regularly approximated with/ɑ(ː)/ in North America and/æ/ in RP. However, in the case of mid back rounded vowels spelled⟨o⟩, the usual North American approximation is/oʊ/, not/ɑː/ (in RP, it can be either/əʊ/ or/ɒ/). However, when the vowel is both stressed and word-final, the only possibilities in RP are/ɑː/ in the first case and/əʊ/ in the latter case, mirroring GA.

For the sake of simplicity, instances of an unroundedLOT vowel (phonetically[ɑ]) that do not merge withPALM/START are excluded from the table below. For this reason, the traditional Norfolk dialect is included but the contemporary one, nor the Cardiff dialect, are not.

In many dialects of English, the vowel/oʊ/ has undergone fronting. The exact phonetic value varies. Dialects with the fronted/oʊ/ include Received Pronunciation; Southern, Midland, and Mid-Atlantic American English; and Australian English. This fronting does not generally occur before/l/, a relatively retracted consonant.

• Phonological history of English
• Phonological history of English vowels

• Barber, Charles Laurence (1997).Early modern English (second ed.). Edinburgh: Edinburgh University Press.ISBN 0-7486-0835-4.
• Labov, William; Ash, 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.
• Ostalski, Przemysław (2009)."Back Vowels in British and American English"(PDF).Przedsiębiorczość I Zarządzanie.5 (4):105–118. Retrieved2 February 2016.
• Wells, John C. (1982).Accents of English. Vol. 1: An Introduction (pp. i–xx, 1–278), Vol. 2: The British Isles (pp. i–xx, 279–466), Vol. 3: Beyond the British Isles (pp. i–xx, 467–674). Cambridge University Press.doi:10.1017/CBO9780511611759, doi:10.1017/CBO9780511611766.ISBN 0-52129719-2 , 0-52128540-2 , 0-52128541-0 .

• Scottish English
• American English
• English language in Canada
• Splits and mergers in English phonology
• Sociolinguistics

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