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: , , , .
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, in which the second element was a high or . Many of these came about through vocalization of the palatal approximant or the labio-velar approximant (which was sometimes from an earlier voiced velar fricative , an allophone of ), 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).
"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" had the same vowel.
The merged vowel was a diphthong, transcribed or . Later (around the 17th century) 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 a front vowel:
and four ended in a back vowel:
Typical spellings are as in the examples above. The spelling "ew" is ambiguous between and , and the spellings "oi" and "oy" are ambiguous between 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 beem reduced as a result of several developments, all of which took place in the mid-to-late 16th century:
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 changes caused only the three 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).
Variation in present-day English.
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.
"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. But in a few regional accents, including some in East Anglia, South Wales, and even Newfoundland, the merger has not gone through (at least not completely), so that 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 this distinction in Norfolk, saying that "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.
"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.)
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 , 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.
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 distinction, with back or in the "toe" set and central in the "tow" set, with the latter but not the former showing the influence of Estuary English.
Walters reports the survival of the distinction in the Welsh English spoken in the Rhondda Valley, with in the "toe" words and 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 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 is usually represented by "oa", "oe", or "oCe" as in "boat", "road", "toe", "doe", "home", "hose", "go", "tone" etc.
This merger did not occur before "r" originally, and only later occurred (relatively recently) as the horse–hoarse merger. This merger is not universal, however, and thusly words with "our" and "oar" may not sound the same as words with "or" in some dialects.
"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 also generally has a merger of and , 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.
"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 . This merger requires the presence of the "father-bother" merger before it can occur.
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.
"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.
"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.
"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, due to an up-gliding vowel.
"Mare"–"mayor" merger.
The "mare"–"mayor" merger occurs in many varieties of British English, in the Philadelphia dialect, and the Baltimore dialect. The process has bisyllabic pronounced as the centering diphthong 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. Particularly in the case of derived from , such words are frequently hypercorrected with . The best-known examples are "mayonnaise" () and "graham" (, a homophone of "gram").