Phonological history of English open back vowels

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.
Overview.
Old and Middle English.
In the Old English vowel system, the vowels in the open back area were unrounded: . There were also rounded back vowels of mid-height: . The corresponding spellings were and , with the length distinctions not normally marked; in modern editions of Old English texts, the long vowels are often written , .
As the Old English (OE) system developed into that of Middle English (ME), the OE short vowel merged with the fronted to become a more central ME . Meanwhile, the OE long vowel was rounded and raised to ME . OE short remained relatively unchanged, becoming a short ME vowel regarded as , while OE long became ME (a higher vowel than ). Alternative developments were also possible; see English historical vowel correspondences for details.
Later, ME open syllable lengthening caused the short vowel to be normally changed to in open syllables. Remaining instances of the short vowel 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:
Unrounded.
In a few varieties of English, the vowel in "lot" is unrounded, pronounced toward []. 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 , therefore being kept distinct from the vowel in "palm", pronounced or . 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 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" () as well as "Saab" and "sob" ().
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 -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.
split.
The split is the result of a late 17th-century sound change that lengthened to before voiceless fricatives, and also before in the word "gone". It was ultimately raised and merged with of words like "thought", although in some accents that vowel is actually open . 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 ; 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 . Accents affected by this change include American English and, originally, RP, although today words of this group almost always have short in RP. The split still exists in some older RP speakers, including Queen Elizabeth II.
The lengthening and raising generally happened before the fricatives , and . In American English the raising was extended to the environment before and , and in a few words before as well, giving pronunciations like for "long", for "dog" and for "chocolate".
In the varieties of American English that have the lot–cloth split, the "lot" vowel is usually symbolized as , often called the "short o" (although from a phonological standpoint it is not a "short" vowel), and the "cloth" vowel as , often called the "open o". The actual pronunciation of these vowels may vary somewhat from the symbol used to denote them; e.g. is often pronounced closer to an open back rounded vowel , and is sometimes fronted to an open central vowel . Some words vary as to which vowel they have. For example, words that end in "-og" like "frog", "hog", "fog", "log", "bog" etc. have rather than in some accents.
There are also significant complexities in the pronunciation of written "o" occurring before one of the triggering phonemes 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 in close syllables. Hence 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 . 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 or ). 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 ), but not the same vowel as "don" etc. (). 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.
The "cot"–"caught" merger (also known as the low back merger or the merger) is a phonemic merger occurring in many English accents, where the vowel sound in words like "cot", "nod", and "stock" (the vowel), has merged with that of "caught", "gnawed", and "stalk" (the vowel). For example, with the merger, "cot" and "caught" become perfect homophones.
Other changes.
split.
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 (possibly even in broad Cockney varieties), and open syllables have a lower vowel quality or a centering diphthong .
Originally-open syllables with an inflectional suffix (such as "bored") retain the lower vowel quality, creating minimal pairs such as "bored" vs. "board" .
In broad Geordie, some words (roughly, those spelled with "a", as in "walk" and "talk") have (which phonetically is the long counterpart of ) instead of the standard . Those are the traditional dialect forms which are being replaced with the standard . is therefore not necessarily a distinct phoneme in the vowel system of Geordie, also because it occurs as an allophone of before voiced consonants.
merger.
The merger is a merger of the English vowels and (with the latter vowel corresponding to 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 merger.
Distribution of /ɑː/.
The distribution of the vowel transcribed with in broad IPA varies greatly among dialects. It corresponds to , , and (when not prevocalic within the same word) and even in other dialects:
In many Scottish dialects, there is just one unrounded open vowel that has two allophones. Those dialects usually do not differentiate from and use for both.
For the sake of simplicity, instances of an unrounded vowel (phonetically ) that do not merge with / 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.
Fronted /oʊ/.
In many dialects of English, the vowel has undergone fronting. The exact phonetic value varies. Dialects with the fronted include Received Pronunciation; Southern, Midland, and Mid-Atlantic American English; and Australian English. This fronting does not generally occur before , a relatively retracted consonant.