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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 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, /iu/ /iːu/, spelt ⟨io⟩ (found in Anglian dialects, but merged into /eo/ /eːo/ in Late West Saxon)
• high, narrower, possibly /iy/ /iːy/ or /ie/ /iːe/, spelt ⟨ie⟩ (found in Late West Saxon)
• mid, /eo/ /eːo/, spelt ⟨eo⟩
• low, /æɑ/ /æːɑ/, spelt ⟨ea⟩

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: ⟨īo⟩, ⟨īe⟩, ⟨ēo⟩, ⟨ēa⟩.

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

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 /j/ (usually from an earlier /ʝ/) or the labio-velar approximant /w/ (sometimes from an earlier voiced velar fricative [ɣ]), when they followed a vowel. For example:

• OE dæg ("day") and weg ("way") (where the /ɡ/ had been palatalized to /j/) became [daj] and [wɛj]
• OE clawu ("claw") and lagu ("law") became [klaw] and [law]

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). For example:

• OE streht ("straight") became [strɛjçt]
• OE þoht ("thought") became [θɒwxt]

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).

Early Middle English had two separate diphthongs /ɛj/ and /aj/. The vowel /ɛj/ was typically represented orthographically with "ei" or "ey", and the vowel /aj/ was typically represented orthographically with "ai" or "ay". These came to be merged, perhaps by the fourteenth century.^[1] 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 /ej/ and /aj/ respectively. Similarly, vein and vain (borrowings from French) were pronounced differently as /vejn/ and /vajn/. After the merger, vein and vain were homophones, and way and day rhymed.

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

The English of southeastern England around 1400 had seven diphthongs,^[2] of which three ended in /j/:

• /ʊj/ as in boil, destroy, coin, join
• /ɛj/ as in nail, day, whey (the product of the vein–vain merger)
• /ɔj/ as in joy, noise, royal, coy

and four ended in /w/:

• /ɪw/ as in view, new, due, use, lute, suit, adieu (the product of a merger of earlier /iw/ and /ew/, and incorporating French loans that originally had /y/)
• /ɛw/ as in few, dew, ewe, shrewd, neuter, beauty
• /ɔw/ as in low, soul
• /ɑw/ 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 /ɪw/ and /ɛw/, and the spellings oi and oy are used for both /ɔj/ and /ʊj/. 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. The diphthongs were /əɪ/ as in tide, and /əʊ/ as in house.^[3] 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:^[4]

• /ɛw/ merged into /ɪw/ and so dew and due became homophones.
• /ɛj/ (from the vein–vain merger) became monophthongized and merged with the /ɛː/ of words like name (which before the Great Vowel Shift had been long /aː/). For more information, see pane–pain merger, below.
  • For a time, many speakers had an /ɛː/ monophthong in pain distinct from an /æː/ monophthong in pane.^[5]
• /ɑw/, as in cause, became monophthongized to /ɒː/, later raising to modern /ɔː/.^[5]
• /ɔw/, as in low, was monophthongized to /ɔː/. That would later rise to /oː/, which merged with the vowel of toe^{[dubious – discuss]}; see toe–tow merger, below.

That left /ɪu/, /ɔɪ/, /ʊɪ/, /əɪ/ and /əʊ/ as the diphthongs of south-eastern England.

By the late 17th century, these further developments had taken place in the dialect of south-eastern England:^[4]

• The falling diphthong /ɪw/ of due and dew changed to a rising diphthong, which became the sequence [juː]. The change did not occur in all dialects, however; see Yod-dropping.
• The diphthongs /əɪ/ and /əʊ/ of tide and house widened to /aɪ/ and /aʊ/, respectively.
• The diphthong /ʊɪ/ merged into /əɪ/~/aɪ/. 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 /ɔɪ/ in the oi words result from regional variants, which had always had /ɔɪ/, rather than /ʊɪ/, perhaps because of influence by the spelling.^[6]

The changes above caused only the diphthongs /aɪ/, /aʊ/ and /ɔɪ/ to remain.

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

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

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

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

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. Zulu English often also has a cot-caught merger, so that sets like "cot", "caught" and "coat" can be homophones.^[7]

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

The line–loin merger is a merger between the diphthongs /aɪ/ and /ɔɪ/ 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.^[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 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^[9] as the long mid mergers. All accents with the pane–pain merger have the toe–tow merger and vice versa.

The pane–pain merger is a merger of the long mid monophthong /eː/ and the diphthong /eɪ/ 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, 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 [eː] and the pain words pronounced with [æɪ], 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 /eː/ to the set of /æɪ/ 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 /æɪ/ — the completion of a slow process of lexical diffusion."^[10]

Walters (2001)^[11] reports the survival of the distinction in the Welsh English spoken in the Rhondda Valley, with [eː] in the pane words and [ɛi] in the pain words.

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. and the phoneme /eː/ 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.

The toe–tow merger is a merger of the Early Modern English vowels /oː/ (as in toe) and /ou/ (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 /ɔː/ and /ɔw/ respectively, and they shifted in the second phase of the Great Vowel Shift.)

The merger occurs in the vast majority of Modern English accents; whether the outcome is monophthongal or diphthongal depends on the accent. 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 and South Wales, 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.

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.^[12]

The distinction is most often preserved in East Anglian accents, especially in Norfolk. Peter Trudgill^[10] 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,^[13] 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.

Walters^[11] reports the survival of the distinction in the Welsh English spoken in the Rhondda Valley, with [oː] in the toe words and [ow] in the tow words.

Reports of Maine English in the 1970s reported a similar toad-towed distinction among older speakers, but was lost in subsequent generations.

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.

The mare–mayor merger occurs in British English and the Philadelphia—Baltimore dialect, and among scattered other American English speakers. The process has bisyllabic /eɪ.ər/ pronounced with a centering diphthong as in /eər/ in many words. Such varieties pronounce mayor as /ˈmeə(r)/, homophonous with mare.

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

The pride–proud merger is a merger of the diphthongs /aɪ/ and /aʊ/ before voiced consonants into monophthongal /a/ 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 /ɑ/, /aɪ/ and /aʊ/ before voiced consonants, making pride, prod, and proud and find, found and fond homophones.^[14]

The rod–ride merger is a merger of /ɑ/ and /aɪ/ occurring for some speakers of Southern American English and African American Vernacular English, in which rod and ride are merged as /rad/.^[14] Some other speakers may keep the contrast, so that rod is /rɑd/ and ride is /rad/.

This is similar to an earlier sound change where Proto-Germanic *ai shifted to Old English ā.

Smoothing of /aɪ.ə/ is a process that occurs in many varieties of British English where bisyllabic /aɪ.ə/ becomes the triphthong /aɪə/ in certain words with /aɪ.ə/. As a result, "scientific" is pronounced /saɪənˈtɪf.ɪk/ with three syllables and "science" is pronounced /ˈsa(ɪ)əns/ with one syllable.^[15]

• Phonological history of English
• Phonological history of English vowels
• Trisyllabic laxing
• Great Vowel Shift