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Canadian raising refers to either of two similar sound changes that occur in a number of North American varieties of the English language, in which certain diphthongs are "raised" before voiceless consonants (e.g., //, //, //, //, //). The first variant, "classic" Canadian raising, occurs largely in Canadian English and in certain nearby areas of the northern United States, and affects both // and //. This results in the stereotypical Canadian pronunciation of about as "aboot". A second variant with a much larger distribution across many parts of the United States affects only //, and results in differing pronunciations of the first vowel in the words rider and writer.
The raised variant of // typically becomes [ʌɪ], while the raised variant of // varies by dialect, with [ʌʊ] more common in the west and a fronted variant [ɛʉ] commonly heard in Central Canada. In any case, the [a]-component of the diphthong changes from a low vowel to a mid-low vowel ([ʌ], [ɐ] or [ɛ]).
Those speakers with either variant will pronounce the words rider and writer as [ˈɹaɪɾɚ] and [ˈɹʌɪɾɚ], respectively, while those speakers lacking the change entirely will pronounce both as [ˈɹaɪɾɚ]. (In Received Pronunciation in the United Kingdom, these words would be pronounced [ˈɹaɪdə] and [ˈɹaɪtə], respectively). This phenomenon preserves the recoverability of the phoneme // in "writer" despite the North American English process of flapping, which merges // and // into [ɾ] before unstressed vowels.
Geographic distribution 
Despite its name, the phenomenon is not restricted to Canada. "Classic" Canadian raising, affecting both the // and // diphthongs, is quite common in New England (including in the traditional accent of Martha's Vineyard), and also occurs in parts of the upper Midwest. Southern Atlantic varieties of English and the accents of the Fens in England feature it as well. The second, "American" variety, affecting only //, can be found in the northern United States, the Mid-Atlantic Dialect region, California, and probably in many other parts of the country, as it appears to be spreading. A portion of the coastal South Atlantic raises // but not //. There are also Canadians who raise // and not // or vice versa.
For many speakers, Canadian raising is not stopped just by any voiced consonants; rather, only voiced consonants that come right before a morpheme boundary stop it. So, the voiced // in "rider" stops the raising, because it is morpheme-final, while the // in "spider" does not, and for these speakers "rider" does not rhyme with "spider". Similarly, "pilot" gets raised because 'l' is non-final, but the 'l' in "pile it" stops the raising—although in such circumstances (before resonant consonants, it seems), the raising may be optional for some speakers. There are many other dialect-specific complexities: For example, even the speakers just described, for whom "rider" and "spider" do not rhyme, may differ on whether raising applies in "hydrogen", although unquestionably it does apply to "nitrogen".
Canadian raising can also apply across word boundaries in idiomatic expressions. Hence, high school [ˈhʌɪskul] as a term meaning "a secondary school for students approximately 14–18 years old" has raising of the vowel in "high", whereas high school [ˈhaɪ ˈskul] with the literal meaning "a school that is high (e.g. in elevation)" is unaffected. (The two terms are also distinguished by the position of the stress accent, as shown).
Possible origins 
Some[who?] have hypothesized that Canadian raising may be related historically to a similar phenomenon that exists in Scots and Scottish English. The Scottish Vowel Length Rule lengthens a wide variety of vowel sounds in several environments, and shortens them in others; "long" environments include when the vowel precedes a number of voiced consonant sounds. This rule also conditions // in the long environments and // in the short environments. Significantly, though, the Scots Vowel Length Rule applies only before voiced fricatives and //, whereas Canadian raising is not limited in this fashion; thus, it may represent a sort of merging of the Scots Vowel Length Rule with the general English rule lengthening vowels before voiced consonants of any sort.
The most common understanding of the Great Vowel Shift is that the Middle English vowels [iː, uː] passed through a stage [əɪ, əʊ] on the way to their modern pronunciations [aɪ, aʊ]. Given its prevalence in areas of North America first settled by native English speakers, it is likely this is not an innovation of "raising" from an underlying vowel quality to another in the least, but, rather, of the preservation of an older vowel quality in a restricted environment.
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- Wells, J. C. Accents of English. Cambridge: Cambridge University Press, 1982.