Both kinds arose from sound changes occurring in Old English itself, although the long forms sometimes also developed from Proto-Germanic diphthongs.
The set of diphthongs that occurred depended on dialect (and their exact pronunciation is in any case uncertain).
For example: Diphthongs also arose as a result of vowel breaking before /h/ (which had allophones [x] and [ç] in this position – for the subsequent disappearance of these sounds, see h-loss).
The most common words with ew pronounced /ɛw/ were dew, few, hew, lewd, mew, newt, pewter, sew, shew (show), shrew, shrewd and strew.
Words in which /ʊj/ was commonly used included boil, coin, destroy, join, moist, point, poison, soil, spoil, Troy, turmoil and voice, although there was significant variation.
[2] By the mid-16th century, the Great Vowel Shift had created two new diphthongs out of the former long close monophthongs /iː/ and /uː/ of Middle English.
By the late 17th century, these further developments had taken place in the dialect of south-eastern England:[4] The changes above caused only the diphthongs /aɪ/, /aʊ/ and /ɔɪ/ to remain.
The cot–coat merger is a phenomenon exhibited by some speakers of Zulu English in which the phonemes /ɒ/ and /oʊ/ are not distinguished, making "cot" and "coat" homophones.
Pairs like line and loin, bile and boil, imply and employ are homophones in merging accents.
[8] The earliest stage of Early Modern English had a contrast between the long mid monophthongs /eː, oː/ (as in pane and toe respectively) and the diphthongs /ɛj, ɔw/ (as in pain and tow respectively).
In accents that preserve the distinction, the phoneme /ei/ is usually represented by the spellings ai, ay, ei and ey as in day, play, rain, pain, maid, rein, they etc.
The traditional phonetic transcription for General American and earlier Received Pronunciation in the 20th century is /oʊ/, a diphthong.
But in a few regional accents, including some in Northern England, East Anglia, South Wales and South Asia, the merger has not gone through (at least not completely), so that pairs like toe and tow, moan and mown, groan and grown, sole and soul, throne and thrown are distinct.
[10] In 19th century England, the distinction was still very widespread; the main areas with the merger were in the northern Home Counties and parts of the Midlands.
In a recent investigation into the English of the Fens,[14] young people in west Norfolk were found to be maintaining the distinction, with back [ʊu] or [ɤʊ] in the toe set and central [ɐʉ] in the tow set, with the latter but not the former showing the influence of Estuary English.
In accents that preserve the distinction, the phoneme descended from Early Modern English /ou/ is usually represented by the spellings ou, and ow as in soul, dough, tow, know, though etc.
or through L-vocalization as in bolt, cold, folk, roll etc., while that descended from Early Modern English /oː/ is usually represented by oa, oe, or oCe as in boat, road, toe, doe, home, hose, go, tone etc.