In the history of English phonology, there have been many diachronic sound changes affecting vowels, especially involving phonemic splits and mergers. A number of these changes are specific to vowels which occur before /l/, especially in cases where the /l/ is at the end of a syllable (or is not followed by a vowel).

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Diphthongization occurred since Early Modern English in certain -al- and -ol- sequences before coronal or velar consonants, or at the end of a word or morpheme. In these sequences, /al/ became /awl/ and then /ɑul/, while /ɔl/ became /ɔwl/ and then /ɔul/. Both of these merged with existing diphthongs: /ɑu/ as in law and /ɔu/ as in throw.

At the end of a word or morpheme, this produced all, ball, call, fall, gall, hall, mall, small, squall, stall, pall, tall, thrall, wall, control, droll, extol, knoll, poll (meaning a survey of people,) roll, scroll, stroll, swollen, toll, and troll. The word shall did not follow this trend, and remains /ʃæl/ today.

Before coronal consonants, this produced Alderney, alter, bald, balderdash, false, falter, halt, malt, palsy, salt, Wald, Walter, bold, cold, fold, gold, hold, molten, mould/mold, old, shoulder (earlier sholder), smolder, told, and wold (in the sense of "tract of land"). As with shall, the word shalt did not follow this trend, and remains /ʃælt/ today.

Before /k/, this produced balk, caulk/calk, chalk, Dundalk, falcon, stalk, talk, walk, folk, Polk, and yolk.

This L-vocalization established a pattern that would influence the spelling pronunciations of some relatively more recent loanwords like Balt, Malta, waltz, Yalta, and polder. It also influenced English spelling reform efforts, explaining the American English mold and molt vs. the traditional mould and moult.

Certain words of more recent origin or coining, however, do not have the change and retain short vowels, including Al, alcohol, bal, Cal, calcium, gal, Hal, mal-, pal, Sal, talc, Val, doll, Moll, and Poll (a nickname for a parrot.)

Some words such as salt, traditionally pronounced by most RP speakers with /ɔːl/ followed by a consonant, have alternative pronunciations with /ɒl/ that are used more frequently by younger British English speakers. This variation between /ɔːl/ and /ɒl/ occurs primarily before voiceless consonants, as in salt, false and alter; less commonly, /ɒl/ may also be used in words where the /l/ comes before a voiced consonant, as in bald, scald and cauldron.^[1][2] In Great Britain, this laxing before /l/ was traditionally associated with Northern England and Wales,^[3] but has in recent decades become more widespread, including among younger speakers of RP.^[2]

More extensive L-vocalization is a notable feature of certain dialects of English, including Cockney, Estuary English, New York English, New Zealand English, Pittsburgh and Philadelphia English, in which an /l/ sound occurring at the end of a word or before a consonant is pronounced as some sort of close back vocoid, e.g., [w], [o] or [ʊ]. The resulting sound may not always be rounded. The precise phonetic quality varies. It can be heard occasionally in the dialect of the English East Midlands, where words ending in -old can be pronounced /oʊd/. KM Petyt (1985) noted this feature in the traditional dialect of West Yorkshire but said it has died out.^[4] However, in recent decades l-vocalization has been spreading outwards from London and the south east,^[5][6] John C Wells argued that it is probable that it will become the standard pronunciation in England over the next one hundred years,^[7] an idea which Petyt criticised in a book review.^[8]

In Cockney, Estuary English and New Zealand English, l-vocalization can be accompanied by phonemic mergers of vowels before the vocalized /l/, so that real, reel and rill, which are distinct in most dialects of English, are homophones as [ɹɪw].

Graham Shorrocks noted extensive L-vocalisation in the dialect of Bolton, Greater Manchester and commented, "many, perhaps, associate such a quality more with Southern dialects, than with Lancashire/Greater Manchester."^[9]

In the accent of Bristol, syllabic /l/ can be vocalized to /o/, resulting in pronunciations like /ˈbɒto/ (for bottle). By hypercorrection, however, some words originally ending in /o/ were given an /l/: the original name of the town was Bristow, but this has been altered by hypercorrection to Bristol.^[10]

African-American English (AAE) dialects may have L-vocalization as well. However, in these dialects, it may be omitted altogether (e.g. fool becomes [fuː]. Some English speakers from San Francisco - particularly those of Asian ancestry - also vocalize or omit /l/.^[11]

The salary–celery merger is a conditioned merger of /æ/ (as in bat) and /ɛ/ (as in bet) when they occur before /l/, thus making salary and celery homophones.^{[12][13][14][15]} The merger is not well studied. It is referred to in various sociolinguistic publications, but usually only as a small section of the larger change undergone by vowels preceding /l/ in articles about l-vocalization.

This merger has been detected in the English spoken in New Zealand and in parts of the Australian state of Victoria, including the capital Melbourne.^[16][17] The merger is also found in the Norfuk dialect spoken on Norfolk Island.^[15] The salary-celery merger is also characteristic of Chicano English in Los Angeles and has been attested in the Chicano English of northern New Mexico and Albuquerque as well.^[18][19][20] /ɛ/ is also often lowered before /l/ in El Paso, but not all speakers show a merger.^[21] In varieties with the merger, salary and celery are both pronounced /sæləri/.^[13]

The study presented by Cox and Palethorpe at a 2003 conference tested just one group of speakers from Victoria: 13 fifteen-year-old girls from a Catholic girls' school in Wangaratta. Their pronunciations were compared with those of school girl groups in the towns of Temora, Junee and Wagga Wagga in New South Wales. In the study conducted by Cox and Palethorpe, the group in Wangaratta exhibited the merger while speakers in Temora, Junee and Wagga Wagga did not.^[13]

Deborah Loakes from Melbourne University has suggested that the salary-celery merger is restricted to Melbourne and southern Victoria, not being found in northern border towns such as Albury-Wodonga or Mildura.^[16]

In the 2003 study Cox and Palethorpe note that the merger appears to only involve lowering of /e/ before /l/, with the reverse not occurring, stating that "There is no evidence in this data of raised /æ/ before /l/ as in 'Elbert' for 'Albert', a phenomenon that has been popularly suggested for Victorians."^[13]

Horsfield (2001) investigates the effects of postvocalic /l/ on the preceding vowels in New Zealand English; her investigation covers all of the New Zealand English vowels and is not specifically tailored to studying mergers and neutralizations, but rather the broader change that occurs across the vowels. She has suggested that further research involving minimal pairs like telly and tally, celery and salary should be done before any firm conclusions are drawn.

A pilot study of the merger was done, which yielded perception and production data from a few New Zealand speakers. The results of the pilot survey suggested that although the merger was not found in the speech of all participants, those who produced a distinction between /æl/ and /el/ also accurately perceived a difference between them; those who merged /æl/ and /el/ were less able to accurately perceive the distinction. The finding has been interesting to some linguists because it concurs with the recent understanding that losing a distinction between two sounds involves losing the ability to produce it as well as to perceive it (Gordon 2002). However, due to the very small number of people participating in the study the results are not conclusive.

The fill–feel merger is a conditioned merger of the vowels /ɪ/ and /iː/ before /l/ that occurs in some accents. In Europe, it is commonly found in Estuary English. Otherwise it is typical of certain accents of American English. The heaviest concentration of the merger is found in, but not necessarily confined to, Southern American English: in North Carolina, eastern Tennessee, northern Alabama, Mississippi, northern and central Louisiana (but not New Orleans), and west-central Texas (Labov, Ash, and Boberg 2006: 69-73). This merger, like many other features of Southern American English, can also be found in AAE.

The same two regions show a closely related merger, namely the fell–fail merger of /ɛ/ and /eɪ/ before /l/ that occurs in some varieties of Southern American English making fell and fail homophones. In addition to North Carolina and Texas, these mergers are found sporadically in other Southern states and in the Midwest and West.^[23][24]

The full–fool merger is a conditioned merger of /ʊ/ and /uː/ before /l/, making pairs like pull/pool and full/fool homophones. The main concentration of the pull–pool merger is in Western Pennsylvania English, centered around Pittsburgh. The merger is less consistently but still noticeably present in some speakers of surrounding Midland American English.^[25] The Atlas of North American English also reports this merger, or near-merger, scattered sporadically throughout Western American English, with particular prevalence in some speakers of urban Utahn, Californian, and New Mexican English.^[26] Accents with L-vocalization, such as New Zealand English, Estuary English and Cockney, may also have the full–fool merger in most cases, but when a suffix beginning with a vowel is appended, the distinction returns: Hence 'pull' and 'pool' are [pʊo], but 'pulling' is /ˈpʊlɪŋ/ whereas 'pooling' remains /ˈpuːlɪŋ/.^[27]

The fill–feel merger and full–fool merger are not unified in American English; they are found in different parts of the country, and very few people show both mergers.^[28]

The hull–hole merger is a conditioned merger of /ʌ/ and /oʊ/ before /l/ occurring for some speakers of English English with l-vocalization. As a result, "hull" and "hole" are homophones as [hɔʊ]. The merger is also mentioned by Labov, Ash, and Boberg (2006: 72) as a merger before /l/ in North American English that might require further study. The latter merger can also involve /ʊ/ or /ə/ before /l/.

The gulf-golf merger is the merger of the diaphonemes /ʌ/ and /ɒ/ before /lC/, where C denotes a consonant. It is attested in Australian English, in which it can co-occur with the Doll-dole merger. In Australian English the result of this 2-3 way merger is [ɔ], the vowel of LOT.^[29]

The doll–dole merger is a conditioned merger, for some Londoners, of /ɒ/ and /əʊ/ before syllable-final (or non-prevocalic) /l/, resulting in homophony between pairs like doll and dole.^[30] The distinction between /ɒ/ and /əʊ/ is maintained in derived forms containing prevocalic /l/, such as d[ɒ]lling herself up vs. d[ɒʊ]ling it out, which means that the underlying vowel is recoverable if the /l/ is morpheme-final, as in doll and dole.^[30] But when the /l/ is followed by a consonant within the same morpheme, as in solve, the distinction is not recoverable; this may be the cause, via hypercorrection, of pronunciations such as [səʊlv] for solve in place of RP [sɒlv].^[30]

The goat split is a process that has affected London dialects, Australian English, and Estuary English.^[33][34] In the first phase of the split, the diphthong of goat /əʊ/ developed an allophone [ɒʊ] before "dark" (nonprevocalic) /l/. Thus goal no longer had the same vowel as goat ([ɡɒʊɫ] vs. [ɡəʊʔ]).^[33] In the second phase, the diphthong [ɒʊ] spread to other forms of affected words. For example, the realization of rolling changed from [ˈɹəʊlɪŋ] to [ˈɹɒʊlɪŋ] on the model of roll [ɹɒʊɫ]. This led to the creation of a minimal pair for some speakers: wholly /ˈhɒʊli/ vs. holy /ˈhəʊli/ and thus to phonemicization of the split. The change from /əʊ/ to /ɒʊ/ in derived forms is not fully consistent; for instance, in cockney, polar is pronounced with the /əʊ/ of goat even though it is derived from pole /ˈpɒʊl/.

In broad Cockney, the phonetic difference between the two phonemes may be rather small and they may be distinguished by nothing more than the openness of the first element, so that goat is pronounced [ɡɐɤʔ] whereas goal is pronounced [ɡaɤ].^[33]

Similar to the Goat split, the Goose vowel has developed contrasting phonetic outcomes before /l/ in some Southeastern English dialects, exhibited by the pair ruler (measuring instrument), pronounced with a fronter vowel that can be transcribed [yː]^[35] or [ʉw],^[36] and ruler ('one who rules'), pronounced with a backer vowel that can be transcribed [uː],^[35] [ʊw] or [oː].^[36] This contrast developed from an allophonic distribution where a back variant of the goose vowel is used before tautosyllabic /l/, as in rule /ˈruːl/ [ˈɹuːɫ], but a fronted variant closer to [yː] is used elsewhere, as in ruler (instrument) /ˈruː.lə/ [ˈɹyː.lə].

This distribution has become complicated by morphology in a way that is leading to a phonemic split in words with pre-vocalic /l/: those where the /l/ is stem-final are pronounced with the phonetically back vowel [uː] (as in ruler (monarch), a morphologically transparent derivative of rule), whereas those where the /l/ is stem-medial are pronounced with a fronted vowel [yː] (as in ruler (measuring instrument), which is treated as an unanalyzable unit). The difference in vowel quality is presumably accompanied by a difference in the pronunciation of the following /l/^[35] ([ɫ] after [uː], [l] after [yː]).

A similar backing change has occurred in many North American dialects,^[37] but this has remained allophonic. For example, in California English, the Goose vowel is realized as a back vowel in words such as school where it is followed by /l/, but is fronted in words where it is not followed by /l/, such as new.^[38]

For some English speakers in the UK, the vowels of goose and thought may be merged before dark syllable-final /l/, which may be caused by the raising of the thought vowel to [oː] or [ʊː] in combination with the backing of the goose vowel before /l/ as part of the Goose split.^[39] This neutralization has been found to exist for clusters of speakers in the southern UK, especially for speakers from areas of the south coast and the Greater London area.^[40]

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The vile–vial merger is where the words in the vile set ending with /-ˈaɪl/ (bile, file, guile, I'll, Kyle, Lyle, mile, Nile, pile, rile, smile, stile, style, tile, vile, while, wile) rhyme with words in the vial set ending with /-ˈaɪəl/ (decrial, denial, dial, espial, Niall, phial, trial, vial, viol).^[41] This merger involves the dephonemicization of schwa that occurs after a vowel and before /l/, causing the vowel-/l/ sequence to be pronounced as either one or two syllables.

This merger may also be encountered with other vowel rhymes too, including:

• /-ˈeɪl/ (jail, sale, tail, etc.) and /-ˈeɪəl/ (betrayal, Jael), usually skewing towards two syllables.
• /-ˈɔɪl/ (coil, soil, etc.) and /-ˈɔɪəl/ (loyal, royal), usually skewing towards two syllables.
• /-ˈiːl/ (ceil, feel, steal, etc.) and /-ˈiːəl/ (real), usually skewing towards two syllables.
• /-ˈɔːl/ (all, drawl, haul, etc.) and /-ˈɔːəl/ (withdrawal), usually skewing towards one syllable.
• /-ˈoʊl/ (bowl, coal, hole, roll, soul, etc.) and /-ˈoʊəl/ (Joel, Noel), usually skewing towards one syllable.
• /-ˈuːl/ (cool, ghoul, mewl, rule, you'll, etc.) and /-ˈuːəl/ (cruel, dual, duel, fuel, gruel, jewel), usually skewing towards one syllable.
• /-ˈaʊl/ (owl, scowl, etc.) and /-ˈaʊəl/ (bowel, dowel, Powell, towel, trowel, vowel), inconsistently skewing towards either one or two syllables. Some words may wander across this boundary even in some non-merging accents, such as owl with /-ˈaʊəl/, and bowel with /-ˈaʊl/.
• In some rhotic accents, /-ˈɜrl/ (girl, hurl, pearl, etc.) and /-ˈɜrəl/ (referral), usually skewing towards two syllables. This historically happened to the word squirrel, which was previously /ˈskwɪrəl/ (and still is in certain accents) but became one syllable /ˈskwɜrl/ in General American today. Some accents with one-syllable squirrel later broke it into two syllables again, as /ˈskwɜrəl/.
• In some rhotic father–bother merged accents, /-ˈɑrl/ (Carl, marl, etc.) and /-ˈɑrəl/ (coral, moral), usually skewing towards two syllables.

For many speakers, the vowels in cake, meet, vote and moot can become centering diphthongs before /l/, leading to pronunciations like [teəl], [tiəl], [toəl] and [tuəl] for tail, teal, toll and tool.

In Cockney, non-prevocalic /ʊl/ (as in bull), /ʉːl/ (as in pool), /əl/ (as in bottle) and /oːl/ (as in call) can all merge with the /oː/ of thought, thus reintroducing the phoneme in the word-final position where, according to one analysis, only /ɔə/ can occur (see thought split): /ˈboː, ˈpoː, ˈbɒtoː, ˈkoː/. The last three words can contrast with the open variety of THOUGHT (which is not distinct from NORTH and FORCE and often also encompasses CURE - see cure-force merger), as in core, bore and paw: /ˈkɔə, ˈbɔə, ˈpɔə/, also in pairs such as stalled /ˈstoːd/ - stored /ˈstɔəd/.

The merger of /əl/, /oːl/ and /oː/ is the most usual and leads to musical being homophonous with music hall as /ˈmjʉːzɪkoː/. Cockney speakers usually regard both syllables of awful as rhyming: /ˈoːfoː/.^[42]

The merger of /oːl/ with /oː/ has been reported to occur in New Zealand English, which does not feature the THOUGHT-split (leading to a larger number of potential homophones).^[43]

In the following list, the only homophonous pairs that are included are those involving /oː/ and /oːl/. As the merger is restricted to non-rhotic accents with close THOUGHT, /oː/ in the fifth and sixth columns is assumed to cover not only THOUGHT but also NORTH and FORCE. In the case of Cockney, the sixth column does not participate in the merger.

There is a large amount of potential homophones involving adjectives with the suffix -able and phrases consisting of a related verb, the indefinite article and the nouns bull, ball and boar. However, they require not only emphatically stressing the verb but also no glottal stop before the indefinite article (e.g. afford a bull/ball/boar cannot be pronounced as [əˌfoːdəˈboː], [əˌfoːdʔəˈboː] nor [əˈfoːdʔəboː]), which makes the homophony between the phrases and the adjectives ending in -able less likely than the homophony between the phrases themselves for speakers who have the merger. Again, phrases involving the noun boar are distinct for speakers with the thought split regardless of stress: [əˌfoːdəˈbɔə, əˌfoːdʔəˈbɔə, əˈfoːdʔəbɔə, əˈfoːdəbɔə] ('afford a boar').

Labov, Ash, and Boberg (2006:73) mention four mergers before /l/ that may be under way in some accents of North American English, and which require more study:^[44]

• /ʊl/ and /oʊl/ (bull vs bowl)
• /ʌl/ and /ɔːl/ (hull vs hall)
• /ʊl/ and /ʌl/ (bull vs hull) (effectively undoing the foot-strut split before /l/)
• /ʌl/ and /oʊl/ (hull vs bowl)

• Phonological history of the English language
• Phonological history of English vowels
• English-language vowel changes before historic r