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.
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:
- /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
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/
- At this time, the short /ɔ/ in dog was lowered to /ɒ/
There were thus two open back monophthongs:
- /ɒ/ as in lot
- /ɔː/ as in cause and (before /r/) in more
and one open back diphthong:
- /ɔu̯/ as in low
By 1700, the following further developments had taken place:
- The diphthong /ɔu̯/ of soul was raised to /ou̯/, and then monophthongized to /oː/, merging with boat (see toe–tow merger). Before /r/, this change was later undone by the horse–hoarse merger except in some varieties, as currently seen in Irish English, Scottish English and African American Vernacular English.
- Short /wa/ was retracted and rounded to /wɒ/. The shift was suppressed before a velar consonant, as in quack, twang, wag, wax, and also was suppressed in swam (the irregular past tense of swim). The change of /wa/ to /wɒ/ did not occur in Mid-Ulster English.
- /ɒ/ had begun to partake in lengthening and raising before a nonprevocalic voiceless fricative. That resulted in words like broth, cost and off having /ɒː/ instead of /ɒ/, and was the start of the LOT–CLOTH split (see further below).
- In words such as change and chamber, 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 in laugh, and half, /ɔː/ was shifted to /æ/ instead, derived from Middle English /a/.
- An unrounded back vowel /ɑː/ developed, found in certain classes of words that had previously had /a/, like start, father and palm.
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.
From the 18th century on, the following changes have occurred:
- In dialects with the lot-cloth split, the /ɒː/ was raised to /ɔː/, merging with the vowel in thought.
- In General American, the lot vowel has become unrounded and merged into /ɑ/ (the father–bother merger).
- In RP, the lot–cloth split has been undone, so that words in the CLOTH set are usually now pronounced /ɒ/.
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.
In a few varieties of English, the vowel in lot is unrounded, pronounced toward [ɑ]. This is found in the following dialects:
- Irish English
- Much of the Caribbean
- The West Country and the West Midlands of England
- Most of North American English Excluding Boston, in which it is typically raised toward /ɔ/, merging with the vowel in thought.
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]. 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.)
The father–bother 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/), Saab and sob (/sɑb/), and balm and bomb (/bɑm/, except for some speakers who reintroduce the historical l into balm).
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. 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, 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 /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", 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 a 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:
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).
The cot–caught 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.
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].
The THOUGHT–GOAT merger is a merger of the English vowels /ɔː/ and /oʊ/; the latter vowel is pronounced as a monophthong [oː] in many Northern English dialects. The merger may occur in Geordie, particularly among females, and also in Bradford English. It has also been reported as a possibility in some Northern Welsh accents, and a possibility (a near-merger) in some African American Vernacular English towards [ɔo~ɔʊ].
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.