Phonological history of English close back vowels

Most dialects of modern English have two close back vowels: the near-close near-back rounded vowel found in words like "foot", and the close back rounded vowel (realized as central in many dialects) found in words like "goose". The vowel , which historically was back, is often central as well. This article discusses the history of these vowels in various dialects of English, focusing in particular on phonemic splits and mergers involving these sounds.
Historical development.
The Old English vowels included a pair of short and long close back vowels, and , both written (the longer vowel is often distinguished as in modern editions of Old English texts). There was also a pair of back vowels of mid-height, and , both of which were written (the longer vowel is often in modern editions).
The same four vowels existed in the Middle English system. The short vowels were still written and , but long came to be spelt as , and as . Generally, the Middle English vowels descended from the corresponding Old English ones, but there were certain alternative developments: see Phonological history of Old English#Changes leading up to Middle and Modern English.
The Middle English open syllable lengthening caused short to be mostly lengthened to (an opener back vowel) in open syllables; this development can be seen in words like "nose". During the Great Vowel Shift, Middle English long was raised to in words like "moon"; Middle English long was diphthongised, becoming the present-day , as in "mouse"; and Middle English of "nose" was raised and later diphthongized, leading to present-day .
At some point, short developed into a lax, near-close near-back rounded vowel, , as found in words like "put". (Similarly, short has become.) According to Roger Lass, the laxing occurred in the 17th century, but other linguists have suggested that it may have taken place much earlier. The short remaining in words like "lot" has also been lowered and, in some accents, unrounded (see open back vowels).
Shortening of to.
In a handful of words, some of which are very common, the vowel was shortened to . In a few of those words, notably "blood" and "flood", the shortening happened early enough that the resulting underwent the ""foot–strut" split" (see next section) and are now pronounced with . Other words that underwent shortening later consistently have , such as "good", "book", and "wool". Still other words, such as "roof", "hoof", and "root", are still in the process of the shift, with some speakers preferring and others preferring in such words, such as in Texan English. For some speakers in Northern England, words ending in "-ook", such as "book" and "cook" still have the long vowel.
– split.
The – split is the split of Middle English short into two distinct phonemes: (as in "foot") and (as in "strut"). The split occurs in most varieties of English, the most notable exceptions being most of Northern England and the English Midlands and some varieties of Hiberno-English. In Welsh English, the split is also absent in parts of North Wales, under influence from Merseyside and Cheshire accents, and south Pembrokeshire, where English replaced Welsh long before it occurred in the rest of Wales.
The origin of the split is the unrounding of in Early Modern English, resulting in the phoneme . Usually, unrounding to did not occur if was preceded by a labial consonant, such as , , , and was followed by , , or , leaving the modern . Because of the inconsistency of the split, "put" and "putt" became a minimal pair that were distinguished as and . The first clear description of the split dates from 1644.
In non-splitting accents, "cut" and "put" rhyme, "putt" and "put" are homophonous as , and "pudding" and "budding" rhyme. However "luck" and "look" may not necessarily be homophones since many accents in the area concerned have "look" as , with the vowel of "goose". In the Coventry area, a schwa is often hypercorrected to , such as for 'button'.
The absence of the split is a less common feature of educated Northern English speech than the absence of the trap–bath split. The absence of the foot–strut split is sometimes stigmatized, and speakers of non-splitting accents may try to introduce it into their speech, which sometimes resultes in hypercorrection, such as by pronouncing "butcher" .
The name "- split" refers to the lexical sets introduced by and identifies the vowel phonemes in the words. From a historical point of view, however, the name is inappropriate because the word "foot" did not have short when the split happened, but it underwent shortening only later.
In modern standard varieties of English, such as Received Pronunciation (RP) and General American (GA), spelling is a reasonably good guide to whether a word is in the or lexical sets. The spellings o (apart from the regular ) and u nearly always indicate the STRUT set (common exceptions are "wolf, woman", "pull, bull, full", "push, bush", "cushion", "puss", "put", "pudding" and "butcher"), and the spellings oo and ould usually indicate the FOOT set (common exceptions are "blood" and "flood"). The spellings of some words changed in accordance with that pattern: wull became wool, and wud became wood. In some recent loanwords such as "Muslim" both pronunciations are found.
– merger.
The – merger or the –schwa merger is a merger of with that occurs in Welsh English and some higher-prestige Northern England English. Also usual in General American, the merger causes minimal pairs such as "unorthodoxy" and "an orthodoxy" to be merged. The phonetic quality of the merged vowel depends on the accent. For instance, merging General American accents have as the stressed variant, as the word-final variant. Elsewhere, the vowel surfaces as or even (GA features the weak vowel merger). That can cause words such as "hubbub" ( in RP) to have two different vowels () even though both syllables contain the same phoneme in both merging and non-merging accents. On the other hand, in Birmingham, Swansea and Miami, at least the non-final variant of the merged vowel is consistently realized as mid-central , with no noticeable difference between the stressed and the unstressed allophones.
The merged vowel is typically written with , regardless of its phonetic realization. That largely matches an older canonical phonetic range of the IPA symbol , which used to be described as covering a vast central area from near-close to near-open .
Because of the unstressed nature of , the merger occurs only in unstressed syllables. Word-finally, the two vowels do not contrast in any accent of English (Middle English , the vowel from which was split, could not occur in that position), and the vowel that occurs in that position approaches (the main allophone of in many accents). However, there is some dialectal variation, with varieties such as broad Cockney using variants that are strikingly more open than in other dialects. It is usually identified as belonging to the phoneme, even in accents without the merger, but native speakers may perceive the phonemic makeup of words such as "comma" to be , rather than . The open variety of occurs even in Northern English dialects (such as Geordie) that have not undergone the foot–strut split, but in Geordie, it can be generalised to other positions and so not only "comma" but also "commas" can be pronounced with in the second syllable, which is rare in other accents. In contemporary General British the final is often mid , rather than open .
All speakers of General American neutralise , and (the vowel) before , which results in an r-colored vowel . GA lacks a truly contrastive phoneme ("furry", "hurry", "letters" and "transfer" (n.), distinguished in RP as , , and all have the same r-colored in GA), and the symbol is used only to facilitate comparisons with other accents. See hurry–furry merger for more information.
Some other minimal pairs apart from "unorthodoxy"–"an orthodoxy" include "unequal" vs. "an equal" as well as "a large untidy room" vs. "a large and tidy room" . However, there are few minimal pairs like that, and their use as such has been criticized by scholars such as Geoff Lindsey because the members of such minimal pairs are structurally different. There also are words for which RP always used in the unstressed syllable, such as "pick-up" or "sawbuck" , which merging accents use the same as the second vowel of "balance". In RP, there is a consistent difference in vowel height; the unstressed vowel in the first two words is a near-open (traditionally written with ), but in "balance", it is a mid .
Development of /juː/.
Earlier Middle English distinguished the close front rounded vowel (occurring in loanwords from Anglo-Norman like "duke") and the diphthongs (occurring in words like "new"), (occurring in words like "few") and (occurring in words like "dew").
In Late Middle English, , , and had merged as . In Early Modern English, merged into as well.
 has remained as such in some Welsh, some northern English and a few American accents. Thus, those varieties of Welsh English keep "threw" distinct from "through" . In most accents, however, the falling diphthong turned into a rising diphthong, which became the sequence . The change had taken place in London by the late 17th century. Depending on the preceding consonant and on the dialect, it either remained as or developed into by the processes of yod-dropping or yod-coalescence. That has caused the standard pronunciations of "duke" (or ), "new" , "few" and "rude" .
– merger.
The – merger is a phenomenon that helps define Scottish English, Northern Irish English, Malaysian English, and Singapore English, in which the modern English phonemes and have merged into a single phoneme. As a result, word pairs like "look" and "Luke" are homophones, plus "good" and "food" and "foot" and "boot" rhyme. 
The history of the merger dates back to two Middle English phonemes: the long vowel /oː/ (which "shoot" traces back to) and the short vowel /u/ (which "put" traces back to). As a result of the Great Vowel Shift, /oː/ raised to /uː/, which continues to be the pronunciation of "shoot" today. Meanwhile, the Middle English /u/ later adjusted to /ʊ/, as "put" is pronounced today. However, the /uː/ of "shoot" next underwent a phonemic split in which some words retained /uː/ (like "mood") while the vowel of other words shortened to /ʊ/ (like "good"). Therefore, the two processes (/oː/→/uː/→/ʊ/ and /uː/→/ʊ/) resulted in a merger of the vowels in certain words, like "good" and "put", to /ʊ/, which is now typical of how all English dialects pronounce those two words. (See the table in the section "– split" above for more information about these early shifts.) The final step, however, was for certain English dialects under the influence of foreign languages (the Scots language influencing Scottish English, for example) to merge the newly united /ʊ/ vowel with the /uː/ vowel (of "mood" and "shoot"): the – merger. Again, this is not an internally motivated phonemic merger but the appliance of different languages' vowel systems to English lexical incidence. The quality of this final merged vowel is usually in Scotland and North Ireland but in Singapore. 
The full–fool merger is a conditioned merger of the same two vowels specifically before , which causes pairs like "pull"/"pool" and "full"/"fool" to be homophones; it appears in many other dialects of English and is particularly gaining attention in several American English varieties.
Other changes.
In Geordie, the vowel undergoes an allophonic split, with the monophthong being used in morphologically-closed syllables (as in "bruise" ) and the diphthong being used in morphologically-open syllables word-finally (as in "brew" ) but also word-internally at the end of a morpheme (as in "brews" ).
Most dialects of English turn into a diphthong, and the monophthongal is in free variation with the diphthongal , particularly in word-internally. Word-finally, diphthongs are more usual.
Compare the identical development of the close front vowel.
The change of to is a process that occurs in many varieties of British English in which bisyllabic has become the diphthong in certain words. As a result, "ruin" is pronounced as monosyllabic and "fluid" is pronounced .