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 /u:/ (realized as central [?:] in many dialects) found in words like goose. The STRUT 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.
In the Old English vowel system, there was a pair of short and long close back vowels, /u/ and /u:/, both written ⟨u⟩ (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, /o/ and /o:/, written ⟨o⟩ (the longer one often ⟨?⟩ in modern editions).
The same four vowels existed in the Middle English system. The short vowels were still spelt ⟨u⟩ and ⟨o⟩, but long /u:/ came to be spelt as ⟨ou⟩, and /o:/ as ⟨oo⟩. Generally the Middle English vowels descended from the corresponding Old English ones, although there were certain alternative developments - see Phonological history of Old English#Changes leading up to Middle and Modern English.
Due to Middle English open syllable lengthening, short /o/ was mostly lengthened to /?:/ (an opener back vowel) in open syllables; this development can be seen in words like nose. In the Great Vowel Shift, ME long /o:/ ended up being raised to /u:/, in words like moon; ME long /u:/ was diphthongized, becoming the present-day /a?/, as in mouse; and the /?:/ of nose was raised and later diphthongized, leading to present-day /o? ~ /.
At some point, short /u/ developed into a lax, near-close near-back rounded vowel, /?/, as found in words like put. (Similarly, short /i/ has become /?/.) According to Roger Lass, this laxing occurred in the 17th century, but other linguists have suggested that it may have taken place much earlier. The short /o/ remaining in words like lot has also been lowered, and unrounded in some accents (see open back vowels).
In a handful of words, including some very common ones, the vowel /u:/ was shortened to /?/. In a few of these words, notably blood and flood, this 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 in the process of the shift today, with some speakers preferring /u:/ and others preferring /?/ in such words; for example in Texas. For some speakers in Northern England, words ending in -ook such as book, cook still have the long /u:/ vowel.
The foot-strut split is the split of Middle English short /u/ into two distinct phonemes /?/ (as in foot) and /?/ (as in strut). The split occurs in most varieties of English, the most notable exceptions being those of Northern England and most of the English Midlands.
The origin of the split is the unrounding of /?/ in Early Modern English, resulting in the phoneme /?/. In general (though with some exceptions), unrounding to /?/ did not occur if /?/ was preceded by a labial consonant (e.g., /p/, /f/, /b/) and followed by /l/, /?/, or /t?/, leaving the modern /?/. Because of the inconsistency of the split, the words put and putt became a minimal pair, distinguished as /p?t/ and /p?t/. The first clear description of the split dates from 1644.
In non-splitting accents, cut and put rhyme, putt and put are homophonous as /p?t/, and pudding and budding rhyme. However luck and look are not necessarily homophones; many accents in the area concerned have look as /lu:k/, with the vowel of goose. In the Coventry area, Schwas are often hyper-corrected to /?/, such as /'b?t?n/ for 'button'.
The absence of this 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, sometimes resulting in hypercorrections such as pronouncing butcher /'b?t/.
The name "foot-strut split" refers to the lexical sets introduced by Wells (1982), and identifies the vowel phonemes in the words. From a historical point of view, this name is inappropriate because the word foot did not have short /?/ at the time the split happened, but underwent shortening at a later time.
|Great Vowel Shift||u:||u:||u:||u||u|
In modern standard varieties of English, e.g. Received Pronunciation (RP) and General American (GA), the spelling is a reasonably good guide to whether a word is in the FOOT or STRUT lexical sets. The spellings o and u nearly always indicate the STRUT set (common exceptions are wolf, woman, pull, bull, full, push, bush, cushion, puss, put, pudding and butcher), while 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 this pattern: e.g. wull became wool and wud became wood. In some recent loan words such as Muslim both pronunciations are found.
The strut-comma merger or the strut-schwa merger is a merger of /?/ with /?/ which occurs in Welsh English and some higher-prestige Northern England English. It is also usual in General American, and it 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, in merging General American accents is the stressed variant, the word-final one is and elsewhere the vowel surfaces as or even (GA features the weak vowel merger). This can cause words such as hubbub ( in RP) to have two different vowels (['h?b?b]), even though both syllables contain the same phoneme in both merging and non-merging accents. On the other hand, in Birmingham, Miami and Swansea at least the non-final variant of the merged vowel is consistently realized as mid central , with no noticeable difference between the stressed and unstressed allophones.
The merged vowel is typically written with ⟨?⟩, regardless of its phonetic realization. To a large extent, this 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 (ME /u/, the vowel from which /?/ was split could not occur in that position) and the vowel which occurs in that position approaches (the main allophone of STRUT in many accents), though there is some dialectal variation, with varieties such as broad Cockney using strikingly more open variants than 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 /'k?m?/ rather than /'k?m?/. This open variety of /?/ occurs even in Northern English dialects (such as Geordie) which have not undergone the foot-strut split, though in Geordie it can be generalized to other positions so that 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 neutralize /?/, /?/ and /?:/ (the NURSE vowel) before /r/, 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 this and using them 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 which in RP always have /?/ in the unstressed syllable, such as pick-up or sawbuck which in merging accents have 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 ⟨?⟩), whereas in balance it is a mid .
|a large untidy room||a large and tidy room||/? 'l?:(r)d? ?n'ta?di 'ru:m/|
Earlier Middle English distinguished the close front rounded vowel /y/ (occurring in loanwords from Anglo-Norman like duke) and the diphthongs /iu/ (occurring in words like new), /eu/ (occurring in words like few) and /?u/ (occurring in words like dew).
By the time of Late Middle English, /y/, /eu/, and /iu/ had merged as /?u/. In Early Modern English, /?u/ merged into /?u/ as well.
This /?u/ has remained as such in some Welsh, northern English, and American accents, so that, in a variety of Welsh English, threw /?r?u/ is distinct from through /?ru:/. In the majority of accents, however, the falling diphthong /?u/ turned into a rising diphthong, which became the sequence /ju:/. This change had taken place in London by the end of the 17th century. Depending on the preceding consonant and on the dialect, this either remained as /ju:/ or developed into /u:/ by the processes of yod-dropping or yod-coalescence. Hence the present-day standard English pronunciations of duke /d(j)u:k/ (or /d?u:k/), new /n(j)u:/, few /fju:/ and rude /ru:d/.
The foot-goose merger is a phenomenon that occurs in Scottish English, Ulster varieties of Hiberno-English, Malaysian English and Singapore English, where the vowels /?/ and /u:/ are merged. As a result, pairs like look/Luke are homophones and good/food and foot/boot rhyme. The quality of the merged vowel is usually [?] or [y] in Scottish English and [u] in Singapore English. The use of the same vowel in "foot" and "goose" in these dialects is not due to phonemic merger, but the appliance of different languages' vowel system to the English lexical incidence. The full-fool merger is a conditioned merger of the same two vowels before /l/, making pairs like pull/pool and full/fool homophones.
In Geordie, the GOOSE vowel undergoes an allophonic split, with the monophthong [u: ~ ?:] being used in morphologically closed syllables (as in bruise [b?u:z ~ b:z]) and the diphthong  being used in morphologically open syllables, not only at the very end of a word (as in brew [b]), but also word-internally at the end of a morpheme (as in brews [bz]).
The change of /u:.?/ to  is a process that occurs in many varieties of British English where bisyllabic /u:.?/ becomes the diphthong  in certain words. As a result, "ruin" is pronounced as monosyllabic ['n] and "fluid" is pronounced ['fld].