Phonological history of English diphthongs

English diphthongs have undergone many changes since the Old and Middle English periods. The sound changes discussed here involved at least one phoneme which historically was a diphthong.

Old English
Old English diphthongs could be short or long. Both kinds arose from sound changes occurring in Old English itself, although the long forms sometimes also developed from Proto-Germanic diphthongs. They were mostly of the height-harmonic type (both elements at the same height) with the second element further back than the first. The set of diphthongs that occurred depended on dialect (and their exact pronunciation is in any case uncertain). Typical diphthongs are considered to have been as follows: As with monophthongs, the length of the diphthongs was not indicated in spelling, but in modern editions of OE texts the long forms are often written with a macron: $⟨io⟩$, $⟨ie⟩$, $⟨eo⟩$, $⟨ea⟩$.
 * high, fully backing,, spelt $⟨īo⟩$ (found in Anglian dialects, but merged into in Late West Saxon)
 * high, narrower, possibly, spelt $⟨īe⟩$ (found in Late West Saxon)
 * mid,, spelt $⟨ēo⟩$
 * low,, spelt $⟨ēa⟩$

In the transition from Old to Middle English, all of these diphthongs generally merged with monophthongs.

Development of new diphthongs
Although the Old English diphthongs merged into monophthongs, Middle English began to develop a new set of diphthongs. Many of these came about through vocalization of the palatal approximant (usually from an earlier ) or the labio-velar approximant  (sometimes from an earlier voiced velar fricative ), when they followed a vowel. For example: Diphthongs also arose as a result of vowel breaking before (which had allophones  and  in this position – for the subsequent disappearance of these sounds, see h-loss). For example: The diphthongs that developed by these processes also came to be used in many loanwords, particularly those from Old French. For a table showing the development of the Middle English diphthongs, see Middle English phonology (diphthong equivalents).
 * OE dæg ("day") and weg ("way") (where the had been palatalized to ) became  and
 * OE clawu ("claw") and lagu ("law") became and
 * OE streht ("straight") became
 * OE þoht ("thought") became

Vein–vain merger
Early Middle English had two separate diphthongs and. The vowel was typically represented orthographically with "ei" or "ey", and the vowel  was typically represented orthographically with "ai" or ay". These came to be merged, perhaps by the fourteenth century. The merger is reflected in all dialects of present-day English.

In early Middle English, before the merger, way and day, which came from Old English weġ and dæġ had and  respectively. Similarly, vein and vain (borrowings from French) were pronounced differently as and. After the merger, vein and vain were homophones, and way and day rhymed.

The merged vowel was a diphthong, something like or. Later (around the 1800s) this diphthong would merge in most dialects with the monophthong of words like pane in the pane–pain merger.

Late Middle English
The English of southeastern England around 1400 had seven diphthongs, of which three ended in :
 * as in boil, destroy, coin, join
 * as in nail, day, whey (the product of the vein–vain merger)
 * as in joy, noise, royal, coy

and four ended in :
 * as in view, new, due, use, lute, suit, adieu (the product of a merger of earlier and, and incorporating French loans that originally had )
 * as in few, dew, ewe, shrewd, neuter, beauty
 * as in low, soul
 * as in cause, law, salt, change, chamber, psalm, half, dance, aunt.

Typical spellings are as in the examples above. The spellings eu and ew are both and, and the spellings oi and oy are used for both  and. The most common words with ew pronounced were dew, few, hew, lewd, mew, newt, pewter, sew, shew (show), shrew, shrewd and strew. Words in which was commonly used included boil, coin, destroy, join, moist, point, poison, soil, spoil, Troy, turmoil and voice, although there was significant variation.

16th century
By the mid-16th century, the Great Vowel Shift had created two new diphthongs out of the former long close monophthongs and  of Middle English. The diphthongs were as in tide, and  as in house. Thus, the English of south-eastern England could then have had nine diphthongs.

By the late 16th century, the inventory of diphthongs had been reduced as a result of several developments, all of which took place in the mid-to-late 16th century:
 * merged into and so dew and due became homophones.
 * (from the vein–vain merger) became monophthongized and merged with the of words like name (which before the Great Vowel Shift had been long ). For more information, see pane–pain merger, below.
 * For a time, many speakers had an monophthong in pain distinct from an  monophthong in pane.
 * , as in cause, became monophthongized to, later raising to modern.
 * , as in low, was monophthongized to . That would later rise to, which merged with the vowel of toe; see toe–tow merger, below.

That left, , , and  as the diphthongs of south-eastern England.

17th century
By the late 17th century, these further developments had taken place in the dialect of south-eastern England:
 * The falling diphthong of due and dew changed to a rising diphthong, which became the sequence . The change did not occur in all dialects, however; see Yod-dropping.
 * The diphthongs and  of tide and house widened to  and, respectively.
 * The diphthong merged into ~. Contemporary literature had frequent rhymes such as Mind–join'd in Congreve, join–line in Pope, child–spoil'd in Swift, toils–smiles in Dryden. The present-day pronunciations with  in the oi words result from regional variants, which had always had, rather than , perhaps because of influence by the spelling.

The changes above caused only the diphthongs, and  to remain.

Later developments
In the 18th century or later, the monophthongs (the products of the pane–pain and toe–tow mergers) became diphthongal in Standard English. That produced the vowels and. In RP, the starting point of the latter diphthong has now become more centralized and is commonly written.

RP has also developed centering diphthongs, , , as a result of breaking before /r/ and the loss of when it is not followed by another vowel (see English-language vowel changes before historic ). They occur in words like near, square and cure.

Present-day RP is thus normally analyzed as having eight diphthongs: the five closing diphthongs, , , , (of face, goat, price, mouth and choice) and the three centering diphthongs , ,. General American does not have the centering diphthongs (at least, not as independent phonemes). For more information, see English phonology (vowels).

Coil–curl merger
The coil–curl or oil–earl merger is a vowel merger that historically occurred in some non-rhotic dialects of American English, making both and  become. This is strongly associated with New York City English and New Orleans English, but only the latter has any modern presence of the feature.

Cot–coat merger
The cot–coat merger is a phenomenon exhibited by some speakers of Zulu English in which the phonemes and  are not distinguished, making "cot" and "coat" homophones. Zulu English often also has a cot-caught merger, so that sets like "cot", "caught" and "coat" can be homophones.

This merger can also be found in some broad Central Belt Scottish English accents. The merger of both sounds into is standard in Central Scots.

Line–loin merger
The line–loin merger is a merger between the diphthongs and  that occurs in some accents of Southern English English, Hiberno-English, Newfoundland English, and Caribbean English. Pairs like line and loin, bile and boil, imply and employ are homophones in merging accents.

Long mid mergers
The earliest stage of Early Modern English had a contrast between the long mid monophthongs (as in pane and toe respectively) and the diphthongs  (as in pain and tow respectively). In the vast majority of Modern English accents these have been merged, so that the pairs pane–pain and toe–tow are homophones. These mergers are grouped together by Wells as the long mid mergers. All accents with the pane–pain merger have the toe–tow merger and vice versa.

The usual outcome of the merger is and, with some dialects having  and. However, a few regional dialects maintain the distinction: East Anglia, south Wales, and in older Northern England, Scottish, Newfoundland, and Maine accents. As late as 1800s 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 accents that preserve the distinction, the diphthong phoneme is usually represented by the spellings ai, ay, ei, and ey as in pain, day, reign, or they; with  is being the spellings ou, ow, or ol as in soul, tow, bolt, or roll. The monophthong phoneme is usually represented by ane, -ange, ae, aCV, ea, and borrowed é and e as in pane, baking, range, Mae, wear, café, and Santa Fe; while  is usually represented by oa, oe, or oCV as in boat, toe, home, or over.

The distinction is most often preserved in East Anglian accents, especially in Norfolk. Peter Trudgill discusses this distinction, and states that "until very recently, all Norfolk English speakers consistently and automatically maintained the nose-knows distinction ... In the 1940s and 1950s, it was therefore a totally unremarkable feature of Norfolk English shared by all speakers, and therefore of no salience whatsoever." In a recent investigation into the English of the Fens, young people in west Norfolk were found to be maintaining the toe–tow distinction, with back or  in the toe set and central  in the tow set. This has tow but not toe showing the influence of Estuary English. However, Trudgill also describes a disappearance of the pane–pain distinction in Norfolk: "This disappearance was being effected by the gradual and variable transfer of lexical items from the set of to the set of  as part of dedialectalisation process, the end-point of which will soon be (a few speakers even today maintain a vestigial and variable distinction) the complete merger of the two lexical sets under  – the completion of a slow process of lexical diffusion."

Walters (2001) reports the survival of the distinction in the Welsh English spoken in the Rhondda Valley, with in the pane words and  in the pain words. Likewise, the Rhondda Valley, has in the toe words and  in the tow words.

Mare–mayor merger
The mare–mayor merger occurs in British English and the Philadelphia&mdash;Baltimore dialect, and among scattered other American English speakers. The process has bisyllabic pronounced with a centering diphthong as in  in many words. Such varieties pronounce mayor as, homophonous with mare.

North American English accents with the merger allow it to affect also sequences without since some words with the  sequence merge with, which is associated with /æ/ tensing before nasal consonants. The best-known examples are mayonnaise, crayon , and Graham (, a homophone of gram).

Pride–proud merger
The pride–proud merger is a merger of the diphthongs and  before voiced consonants into monophthongal  occurring for some speakers of African American Vernacular English; making pride and proud, dine and down, find and found, etc. homophones. Some speakers with this merger may also have the rod–ride merger hence having a three–way merger of, and  before voiced consonants, making pride, prod, and proud and find, found and fond homophones.

Rod–ride merger
The rod–ride merger is a merger of and  occurring for some speakers of Southern American English and African American Vernacular English, in which rod and ride are merged as. Some other speakers may keep the contrast, so that rod is and ride is.

Smoothing of
Smoothing of is a process that occurs in many varieties of British English where bisyllabic  becomes the triphthong  in certain words with. As a result, "scientific" is pronounced with three syllables and "science" is pronounced  with one syllable.