The phonological history of English diphthongs comprises sound changes to phonemes that have historically been diphthongs in the English language.
Background 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: high, fully backing, , spelt (found in Anglian dialects, but merged into in Late West Saxon) high, narrower, possibly , spelt (found in Late West Saxon) mid, , spelt low, , spelt 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: , , , .
In the transition from Old to Middle English, all of these diphthongs generally merged with monophthongs.
Middle English
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: OE dæg ("day") and weg ("way") (where the had been palatalized to ) became and OE clawu ("claw") and lagu ("law") became and 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: OE streht ("straight") became OE þoht ("thought") became 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).
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.
Modern English 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 diphthong split into and , though this sound change was still ongoing in the late 18th century. 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 historical ). 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).
Variation in present-day English
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 (sometimes also called the - merger, using the respective lexical set words) is a merger between the diphthongs and that occurs in some accents of Southern English English, Hiberno-English, Newfoundland English, Caribbean English, West Country English, and Northern England English. Pairs like line and loin, bile and boil, imply and employ are homophones in merging accents.
Homophonous pairs IPA aisle oil ally alloy bile boil buy, by, bye, buy boy, buoy divide devoid dried droid file foil fire foyer grind groined guy goy heist hoist hi, high hoy I oi, oy I'll, isle oil imply employ Jain join Kai coy, koi kine coin Kyle coil liar lawyer lied Lloyd line loin Lyle loyal lyre lawyer mile moil nighs noise Nile noil pie poi pies poise pint point ply ploy psi soy quite quoit ride roid rile roil rile royal rye Roy sigh soy sire sawyer sire soya Thai, tie toy tide toyed tile toil try Troy vice voice vied void wry Roy
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.
Pane–pain merger The pane–pain merger is a merger of the long mid monophthong and the diphthong that occurs in most dialects of English. In the vast majority of Modern English accents, the vowels have been merged; whether the outcome is monophthongal or diphthongal depends on the accent. However, in a few regional accents, including some in East Anglia, South Wales, South Asia, and even Newfoundland and older Maine accents, the merger has not gone through (at least not completely) and so pairs like pane-pain are distinct.
A distinction, with the pane words pronounced with and the pain words pronounced with , survived in Norfolk English into the 20th century. Trudgill describes the disappearance of the 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.
In accents that preserve the distinction, the phoneme is usually represented by the spellings ai, ay, ei and ey as in day, play, rain, pain, maid, rein, they etc. and the phoneme is usually represented by aCe as in pane, plane, lane, late etc. and sometimes by é and e as in re, café', Santa Fe etc.
Homophonous pairs IPA ade aid ale ail ate eight bale bail bade bayed blare Blair blaze Blaise cane Cain clade clayed Clare Claire bate bait Daly daily Dane deign daze days e'er air e'er heir ere air ere heir fane fain fare fair faze fays flare flair gale Gail gate gait gaze gays glave glaive grade grayed graze grays hale hail hare hair haze hays haze heys lade laid lane lain laze lays made maid Mae May male mail mane main maze maize maze Mays page Paige pale pail pane pain pare pair pear pair phase fays phrase frays plane plain plate plait Rae ray raze raise raze rays razor raiser re ray sale sail sane sain sane seine sane Seine spade spayed stare stair suede swayed tale tail there their there they're trade trayed vale vail vale veil vane vain vane vein wade weighed wale wail wales wails Wales wails wane wain wane Wayne waste waist wave waive waver waiver whale wail
Toe–tow merger The toe–tow merger is a merger of the Early Modern English vowels (as in toe) and (as in tow) that occurs in most dialects of English. (The vowels in Middle English and at the beginning of the Early Modern English period were and respectively, and they shifted in the second phase of the Great Vowel Shift.)