Phonological history of English open back vowels

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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[edit]

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:[1]

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

16th-century changes[edit]

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. Before non-prevocalic /r/, the raising did not take place, so more was still /mɔːr/.
  • The diphthong /au̯/ found in words such as cause, law, all, salt, psalm, half, change, chamber, dance had become an open back monophthong /ɔː/ or /ɑː/.
  • At this time, the short /ɔ/ in dog was lowered to /ɒ/

There were thus two open back monophthongs:

  • /ɒ/ as in lot
  • /ɔː/ or /ɑː/ as in cause and (before /r/) in more

and one open back diphthong:

  • /ɔu̯/ as in low

17th-century changes[edit]

By 1700, the following further developments had taken place:

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

  • /ɒ/ in lot and want.
  • /ɔː/ in more, cause, and corn.
  • /ɒː/ in cloth and cost.
  • /ɑː/ in start, father and palm.

Later changes[edit]

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:
    • In General American and old-fashioned RP, /ɒː/ was raised to /ɔː/, merging with the vowel in THOUGHT (the cloth-thought merger).
    • In many accents of England, the lengthening of the CLOTH set was undone, restoring the short pronunciation /ɒ/. This became standard RP by the mid-20th century.
  • In General American, the lot vowel has become unrounded and merged into /ɑ/ (the father–bother merger).

This leaves RP with three back vowels:

  • /ɒ/ in lot, want, cloth, and cost.
  • /ɔː/ in more, cause, and corn.
  • /ɑː/ in start, father, and palm.

and General American with two:

  • /ɑ/ in lot, want, start, father, and palm.
  • /ɔ/ in more, cause, corn, cloth and cost.
Lexical changes in cot–caught merger dialects
Lexical set Example words Change GenAm phonemes Minimal pairs IPA Change Cot–caught merger dialects
PALM ah, father, spa Father-bother
/ɑ/ cot, collar, stock,
wok, chock, Don
/kɑt/, /ˈkɑlər/, /stɑk/,
/wɑk/, /tʃɑk/, /dɑn/
/kɑt/, /ˈkɑlər/, /stɑk/,
/wɑk/, /tʃɑk/, /dɑn/
LOT bother, lot, wasp
CLOTH boss, cloth, dog, off Cloth-thought
/ɔ/ caught, caller, stalk,
walk, chalk, dawn
/kɔt/, /ˈkɔlər/, /stɔk/,
/wɔk/, /tʃɔk/, /dɔn/
THOUGHT all, thought, flaunt

Unrounded LOT[edit]

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.[citation needed]

In such accents, lot typically is pronounced as [lɑt],[2] 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[edit]

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/).[3]

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.[4][5]

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.[4][5]

LOT–CLOTH split[edit]

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.[6][7][8][9][10][11][12][13][14] A further list of words is mentioned in the table below:

Rounded and unrounded vowels with lot–cloth splits
Set Rounded /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/).[citation needed] 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).[15]

Cot–caught merger[edit]

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[edit]

GOAT–THOUGHT merger[edit]

The GOAT–THOUGHT merger is a merger of the English vowels /oʊ/ and /ɔː/ that occurs in Bradford English and sometimes also in Geordie and Northern Welsh English.

THOUGHT–FOOT merger[edit]

The THOUGHT–FOOT merger is a merger of the English vowels /ɔː/ and /ʊ/ that occurs in morphologically closed syllables in cockney, rendering fought homophonous with foot as [ˈfʊt]. It is possible only in fast speech.

THOUGHT–GOOSE merger[edit]

The THOUGHT–GOOSE merger is a merger of the English vowels /ɔː/ and /uː/ that occurs in cockney, rendering hoard homophonous with who'd as [ˈʊd], with the vowel quality that is typical of FOOT. It is possible only in fast speech and, in the case of /ɔː/ (but not /uː/), only in morphologically closed syllables.

THOUGHT split[edit]

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].[16]

In broad Geordie, some THOUGHT words (roughly, those spelled with a, as in walk and talk) have [] (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]. [] 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.[17]

Distribution of /ɑː/[edit]

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 the trap–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 the father–bother merger.
  • In GA (which also features the father–bother merger), /ɑː/ mostly corresponds to /ɒ/ in RP.
  • Many speakers in the US and most speakers in Canada use /ɑː/ not only for RP /ɒ/ but also for /ɔː/. Those dialects have the cot–caught merger in addition to the father-bother merger (though a tiny minority of speakers lack the latter merger, like Scottish English).
  • In loanwords, the open central unrounded vowel [ä] in the source language is regularly approximated with /ɑː/ 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.

In many Scottish dialects, there is just one unrounded open vowel /a/ that has two allophones. Those dialects usually do not differentiate /ɒ/ from /ɔː/ and use [ɔ] for both.

For the sake of simplicity, instances of an unrounded LOT vowel (phonetically [ɑ]) that do not merge with PALM/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.

/ɑː/ in native words and non-recent loanwords
Variety Rhotic Mergers and splits Possible spellings
/ɒrV-ɑːrV/ merger card-cord merger cot-caught merger father-bother merger father-farther merger god-guard merger lot-cloth split trap-palm merger trap-bath split ⟨a⟩ ⟨ar⟩ ⟨au⟩ ⟨aw⟩ ⟨o⟩ ⟨or⟩
Australian English no no no no no yes no no no variable yes yes no no no no
Canadian English yes no no yes yes N/A N/A N/A no no yes no yes yes yes no
General American yes no no variable yes N/A N/A yes no no yes no variable variable yes no
Hiberno-English yes no no variable no N/A N/A variable no variable yes no no no no no
New York City English variable possible no no variable variable variable yes no no yes no no no variable no
New Zealand English mostly no no no no no mostly yes no no no yes yes mostly yes no no no no
Northeastern New England English no no no yes no yes no N/A no no yes yes no no no no
Northern England English no no no no no yes no no no no yes yes no no no no
Philadelphia English yes possible no no yes N/A N/A yes no no yes no no no yes no
Received Pronunciation no no no no no yes no no no yes yes yes no no no no
Scottish English variable no no variable no no no (unclear) variable mostly no yes no no no no no
South African English mostly no no no no no mostly yes no variable no yes yes mostly yes no no no no
Southern American English variable mostly no mostly no variable yes variable variable yes no no yes variable variable variable yes mostly no
Traditional Norfolk dialect no variable no no variable yes variable yes no yes variable variable no no yes no
Welsh English mostly no no no no no mostly yes no no no no yes yes no no no no

Fronted /oʊ/[edit]

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 English ɔ ɔ a
Quality change ɒ ɒ
Thought-monophthonging ɔː
Pre-fricative lengthening ɒː
Quality change ɑː
Lot-unrounding ɑ
Loss of distinctive length ɔ ɒ (ɑ) ɑ
Cloththought merger (ɔ) ɔ
General American output ɔ ɔ ɑ ɑ
Cotcaught merger ɑ

See also[edit]


  1. ^ Barber (1997), pp. 108, 111.
  2. ^ Wells (1982), pp. 245, 339–40, 419.
  3. ^ Labov, Ash & Boberg (2006), p. 169.
  4. ^ a b Wells (1982), pp. 136–37, 203–6, 234, 245–47, 339–40, 400, 419, 443, 576.
  5. ^ a b Labov, Ash & Boberg (2006), p. 171.
  6. ^ "possible". Merriam-Webster Dictionary.
  7. ^ "jostle". Merriam-Webster Dictionary.
  8. ^ "impostor". Merriam-Webster Dictionary.
  9. ^ "profit". Merriam-Webster Dictionary.
  10. ^ "Gothic". Merriam-Webster Dictionary.
  11. ^ "bongo". Merriam-Webster Dictionary.
  12. ^ "Congo". Merriam-Webster Dictionary.
  13. ^ "Boston". Merriam-Webster Dictionary.
  14. ^ "foster". Merriam-Webster Dictionary.
  15. ^ Labov, Ash & Boberg (2006), p. 189.
  16. ^ Ostalski (2009), pp. 106–107.
  17. ^ Wells (1982), pp. 360, 375.