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 in some dialects 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 ⟨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 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 four 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: There's also evidence for it in South East England as early as the late 16th century and as late as the 19th century. 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 outside of North America, 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 a phonemic merger of the lexical sets and. It represents unrounded lot, as detailed above, 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. This causes father and bother to become rhymes. This occurs in the great majority of North American accents; of the 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. Examples of possible homophones resulting from the merger include Khan and con as well as Saab and sob. 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.

split

The ** split** is the result of a late 17th-century sound change that lengthened to before voiceless fricatives, and also before in the words gone and sometimes on. It was ultimately raised and merged with of words like thought, although in some accents that vowel is actually open. This means that is not a separate vowel; rather, it means "either or, 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 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. Accents affected by this change include American English accents that lack the cot-caught merger and formerly RP, although with the exception of water, today words of this group almost always have short in RP. The lengthening and raising generally happened before the fricatives, , and , and in the word water for an unknown reason (compare the broadening of a in father). In American English, the raising was extended to the environment before velars and, and sometimes 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" for historical reasons, as the corresponding RP vowel is still short (and it contrasts with as in father and start). The thought vowel is usually transcribed as and it is often called the "open o". Its actual phonetic realization may be open, whereas the lot vowel may be realized as central. Some words vary as to which vowel they have. For example, words that end in -og like frog, hog, fog, log, bog etc. have in some accents and in others. There are also significant complexities in the pronunciation of written o occurring before one of the triggering phonemes 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 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 closed syllables. Hence occurs in crossing, crosser, crosses because it occurs in cross. In contrast, possible, jostle, impostor, profit, Gothic, 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, the word on, which in Northern American English dialects without the cot-caught merger is pronounced, rhyming with don, but in Midland and Southern American English without the merger is pronounced , rhyming with dawn. The isogloss for this difference, termed the ON line, lies between New York City and Philadelphia on the East Coast and runs West as far as speakers without the merger can be found. Pronunciation of the word want as is also strongly associated with the South.

Cot–caught merger

The cot–caught merger (also known as the low back merger or the lexical set merger) is a phonemic merger occurring in many accents of English, 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

merger

The ** merger** is a** merger** of the English vowels of and that has been reported in Geordie since the late 20th century, with a quality around [oː]. The merger is more common among younger female speakers. The merger also exists among older speakers in Bradford English with a quality around [ɔː], but younger speakers are more likely to resist the merger by fronting the vowel.

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 vowel of into the vowel of that appears in Received Pronunciation and some speakers of New Zealand English, occurring only when the vowel appears after wr (historical ).

Distribution of /ɑː/

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

Table