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|Sound change and alternation|
Vowel breaking may be unconditioned or conditioned. It may be triggered by the presence of another sound or by stress, or it may be triggered in no particular way.
Sometimes vowel breaking is defined as a subtype of diphthongization; then, it refers to harmonic (assimilatory) process that involves diphthongization triggered by a following vowel or consonant.
The original pure vowel typically breaks into two segments, and the first segment matches the original vowel and the second segment is harmonic with the nature of the triggering vowel or consonant. For example, the second segment may be /u/ (a back vowel) if the following vowel or consonant is back (such as velar or pharyngeal), and the second segment may be /i/ (a front vowel) if the following vowel or consonant is front (such as palatal).
Thus, vowel breaking, in this restricted sense, can be viewed as an example of assimilation of a vowel to a following vowel or consonant.
Sometimes vowel breaking occurs only in stressed syllables. For instance, Vulgar Latin open-mid /ɛ/ and /ɔ/ changed to diphthongs only when stressed.
Vowel breaking is a very common sound change in the history of the English language, occurring at least three times (with some varieties adding a fourth) listed here in reverse chronological order:
Vowel breaking is characteristic of the "Southern drawl" of Southern American English, where the short front vowels have developed a glide up to [j], and then in some areas back down to schwa: pat [pæjət], pet [pɛjət], pit [pɪjət].
The Great Vowel Shift changed the long vowels /iː uː/ to diphthongs. They became Modern English /aɪ aʊ/.
In early Middle English, a vowel /i/ was inserted between a front vowel and a following /h/ (pronounced [ç] in this context), and a vowel /u/ was inserted between a back vowel and a following /h/ (pronounced [x] in this context). This is a prototypical example of the narrow sense of "vowel breaking" as described above: The original vowel breaks into a diphthong that assimilates to the following consonant, gaining a front /i/ before a palatal consonant and /u/ before a velar consonant.
In Old English, two forms of harmonic vowel breaking occurred: breaking and retraction, and back mutation.
In prehistoric Old English, breaking and retraction changed stressed short and long front vowels i, e, æ to short and long diphthongs spelled io, eo, ea when followed by h or by r, l + another consonant (short vowels only), and sometimes w (only for certain short vowels).
In late prehistoric Old English, back mutation changed short front i, e, æ to short diphthongs spelled io, eo, ea before a back vowel in the next syllable, if the intervening consonant is of a certain nature. The specific nature of which consonants trigger back umlaut and which block them varies from dialect to dialect.
Vowel breaking is present in Scottish Gaelic with the following changes occurring often but variably between dialects: Archaic Irish eː → Scottish Gaelic iə and Archaic Irish oː → Scottish Gaelic uə  Specifically, central dialects have more vowel breaking than others.
Many Romance languages underwent vowel breaking. The Vulgar Latin open vowels e /ɛ/ and o /ɔ/ in stressed position underwent breaking only in open syllables in French and Italian, but in both open and closed syllables in Spanish. Vowel breaking was completely absent in Portuguese and Catalan. The result of breaking varies between languages: e and o became ie and ue in Spanish, ie and uo in Italian, and ie and eu /ø/ in French.
In the table below, words with breaking are bolded.
|Open||petram, focum||piedra, fuego||pierre, feu||pietra, fuoco||pedra, fogo||pedra, foc|
|Closed||festam, portam||fiesta, puerta||fête, porte||festa, porta||festa, porta||festa, porta|
Romanian underwent the general Romance breaking only with /ɛ/, as it did not have /ɔ/:
It underwent a later breaking of stressed e and o to ea and oa before a mid or open vowel:
Sometimes a word underwent both forms of breaking in succession:
The diphthongs that resulted from the Romance and the Romanian breakings were modified when they occurred after palatalized consonants.
In Quebec French, long vowels are generally diphthongized in the last syllable.
Some scholars believe that Proto-Indo-European (PIE) i, u had a kind of breaking before an original laryngeal in Greek, Armenian and Tocharian, whereas the other Indo-European languages have monophthongs:
However, the hypothesis has not been widely adopted.