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Phonological history of English open back vowels

sound change
History and description of
English pronunciation
Historical stages
  • Old English
  • Middle English
General development
  • Overview
  • In Old English
  • In Scots
Development of vowels
  • Overview
  • Great Vowel Shift
  • Close front
  • A
  • Open back
  • Close back
  • Diphthongs
  • Pre-L
  • Pre-R
Development of consonants
  • Single consonants
  • Clusters
Variable features
  • Rhoticity
  • Flapping
  • L-vocalization
  • T-glottalization
  • Cot–caught merger
  • H-dropping
  • Drawl
  • TH
  • R
  • WH
Related topics
  • History of English
  • Spelling
[]This article contains IPA phonetic symbols. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Unicode characters. For an introductory guide on IPA symbols, see Help:IPA.

The phonology of the open back vowels of the English language has undergone changes both overall and with regional variations, through Old and Middle English to the present. The sounds heard in modern English were significantly influenced by the Great Vowel Shift, as well as more recent developments such as the cot–caught merger.


Old and Middle English

In the Old 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/. The corresponding 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 of Middle 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; see English historical vowel correspondences for details.

Later, ME open syllable lengthening caused the short vowel /o/ to be normally changed to /ɔː/ in open 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:

16th-century changes

By 1600, the following changes had occurred:

There were thus two open back monophthongs:

and one open back diphthong:

17th-century changes

By 1700, the following further developments had taken place:

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

Later changes

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

This leaves RP with three back vowels:

and General American with two:

Lexical changes in cot–caught merger dialects
Lexical setExample wordsChangeGenAm phonemesMinimal pairsIPAChangeCot–caught merger dialects
PALMah, father, spaFather-bother
/ɑ/cot, collar, stock,
wok, chock, Don
/kɑt/, /ˈkɑlər/, /stɑk/,
/wɑk/, /tʃɑk/, /dɑn/
/kɑt/, /ˈkɑlər/, /stɑk/,
/wɑk/, /tʃɑk/, /dɑn/
LOTbother, lot, wasp
CLOTHboss, cloth, dog, offCloth-thought
/ɔ/caught, caller, stalk,
walk, chalk, dawn
/kɔt/, /ˈkɔlər/, /stɔk/,
/wɔk/, /tʃɔk/, /dɔn/
THOUGHTall, thought, flaunt

Unrounded LOT

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

Linguists disagree as to whether the unrounding of the lot 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.

In such accents, lot typically is pronounced as [lɑt], therefore being kept distinct from the vowel in palm, pronounced [pɑːm] or [paːm]. However, the major exception to this is North American English, where the vowel is lengthened to merge with the vowel in palm, as described below. This merger is called the LOT–PALM merger or more commonly the father–bother merger. (See further below.)

Father–bother merger

The father–bother merger is unrounded lot taken a step further. On top of being unrounded, the length distinction between the vowel in lot and bother and the vowel in palm and father is lost, so that the two groups merge.

Examples of possible homophones resulting from the merger include Khan and con (/kɑn/) as well as Saab and sob (/sɑb/).

Out of North American dialects that have unrounded lot, the only notable exception to the merger is New York City English, where the opposition with the [ɑ]-type vowel is somewhat tenuous.

While the accents in northeastern New England, such as the Boston accent, also remain unmerged, lot remains rounded and merges instead with cloth and thought, though the outcome of that is still a longish free vowel that is heard as thought by British speakers.


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. It was ultimately raised and merged with /ɔː/ of words like thought, although in some accents that vowel is actually open [ɒː]. 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. Similar changes took place in words with ⟨a⟩; see trap–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 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 /ˈtʃɔklət/ for chocolate.

In the varieties of American English that have the lot–cloth split, the lot vowel is usually symbolized as /ɑ/, often called the "short o" (although from a phonological standpoint it is not a "short" vowel), and the cloth vowel as /ɔ/, often called the "open o". The actual pronunciation of these vowels may vary somewhat from the symbol used to denote them; e.g. /ɔ/ is often pronounced closer to an open back rounded vowel [ɒ], and /ɑ/ is sometimes fronted to an open 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. However, the use of the open o as opposed to the short o is largely predictable. Just like with /æ/-tensing and the trap–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/ in close syllables. Hence /ɔ/ occurs in crossing, crosser, crosses because it occurs in cross; likewise in longing, longer, longest because it occurs in long. However, possible, jostle, impostor, profit, Gothic, bongo, Congo, and boggle all have /ɑ/. However, there are still exceptions in words like Boston and foster. A further list of words is mentioned in the table below:

Rounded and unrounded vowels with lot–cloth splits
SetRounded /ɔ/Unrounded /ɑ/
/-f/all words in this set (off, office, etc.)
/-s/loss, boss, etc.possible, jostle
/-st/Boston, foster, lost, etc.roster
/-ŋ/long, longest, etc.bongo, Congo

Some words may vary depending on the speaker like (coffee, offer, donkey, soggy, boondoggle, etc. with either /ɑ/ or /ɔ/). Meanwhile, other words vary by region. For example, in the Mid-Atlantic U.S. dialect, most famously spoken in metropolitan Philadelphia and Baltimore, the single word on has the same vowel as dawn (in the mid-Atlantic, this is [ɔə~oə]), but not the same vowel as don etc. ([ɑ~ä]). Labov et al. regard this phenomenon as occurring not just in the Mid-Atlantic region, but in all regions south of a geographic boundary that they identify as the ON line, which is significant because it distinguishes most varieties of Northern American English (in which on and Don are closer rhymes) from most varieties of Midland and Southern American English (in which on and dawn are closer rhymes).

Cot–caught merger

Main article: Cot–caught merger

The cotcaught merger (also known as the low back merger or the LOT–THOUGHT merger) is a phonemic merger occurring in many English accents, where the vowel sound in words like cot, nod, and stock (the LOT vowel), has merged with that of caught, gnawed, and stalk (the THOUGHT vowel). For example, with the merger, cot and caught become perfect homophones.

Other changes


In some London accents of English, the vowel in words such as thought, force, and north, 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 as bored) retain the lower vowel quality, creating minimal pairs such as bored [bɔəd] vs. board [boːd].

In broad Geordie, some THOUGHT words (roughly, those spelled with a, as in walk and talk) have [aː] (which phonetically is the long counterpart of TRAP /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.


The THOUGHT–GOAT merger is a merger of the English vowels /ɔː/ and /oː/ (with the latter vowel corresponding to /əʊ/ in RP). It occurs in certain non-rhotic varieties of British English, such as Bradford English and Geordie (particularly among females). It has also been reported as a possibility in some Northern Welsh accents.

It is more accurately called the THOUGHT–GOAT–NORTH–FORCE merger.

Homophonous pairs
Paulpole, Poleˈpɔːl

Distribution of /ɑː/

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 many Scottish dialects, there is just one unrounded open vowel, usually transcribed with ⟨a⟩. Those dialects usually do not differentiate /ɒ/ from /ɔː/ and use [ɔ] for both.

/ɑː/ in native words and non-recent loanwords
VarietyRhoticMergers and splitsPossible spellings
card-cord mergercot-caught mergerfather-bother mergerfather-farther mergerlot-cloth splittrap-palm mergertrap-bath split⟨a⟩⟨ar⟩⟨au⟩⟨aw⟩⟨o⟩⟨or⟩
Australian Englishnonononoyesnonovariableyesyesnononono
Canadian EnglishyesnoyesyesN/AN/Anonoyesnoyesyesyesno
General AmericanyesnovariableyesN/Ayesnonoyesnovariablevariableyesno
New York City Englishvariablepossible when prevocalicnovariablenoyesnonoyesnononovariableno
New Zealand Englishvariablenononovariablenonoyesyesvariablenononono
Northeastern New England EnglishnonoyesnoyesN/Anonoyesyesnononono
Northern England Englishnonononoyesnononoyesyesnononono
Philadelphia Englishyespossible when prevocalicnoyesN/Ayesnonoyesnononoyesno
Received Pronunciationnonononoyesnonoyesyesyesnononono
Scottish EnglishyesnoyesnoN/AN/AyesN/Ayesnonononono
South African Englishmostly nonononomostly yesvariablenoyesyesmostly yesnononono
Southern American Englishvariablemostly novariableyesvariableyesnonoyesvariablevariablevariableyesmostly no
Traditional Norfolk dialectnonononoyesyesnoyesnonononoyesno
Welsh Englishmostly nonononomostly yesnononoyesyesnononono

Fronted /oʊ/

See also: English-language vowel changes before historic /l/ § Goat split

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.


Stages leading to some of the open back vowels of General American, summarized from Wells (1982), with the cotcaught merger added
Middle Englishɔɔa
Quality changeɒɒ
Pre-fricative lengtheningɒː
Quality changeɑː
Loss of distinctive lengthɔɒ(ɑ)ɑ
Cloththought merger(ɔ)ɔ
General American Outputɔɔɑɑ
Cotcaught mergerɑ

Other topics

This article uses material from the Wikipedia article Phonological history of English open back vowels, which is released under the Creative Commons Attribution-ShareAlike 3.0 Unported License (view authors).

Date of last edit: 2021-01-29T12:58:03.000Z