Vowel breaking may be unconditioned or conditioned. It may be triggered by the presence of another sound, by stress, or in no particular way.
Vowel breaking is sometimes defined as a subtype of diphthongization, when 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. 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 the restricted sense, can be viewed as an example of assimilation of a vowel to a following vowel or consonant.
Vowel breaking is sometimes not assimilatory and is then not triggered by a neighboring sound. That was the case with the Great Vowel Shift in English in which all cases of /i:/ and /u:/ changed to diphthongs.
Vowel breaking sometimes occurs only in stressed syllables. For instance, Vulgar Latin open-mid /?/ and /?/ changed to diphthongs only when they were 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 [pæj?t], pet [p?j?t], pit [p?j?t].
The Great Vowel Shift changed the long vowels /i: u:/ to diphthongs, which became Modern English /a? a?/.
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).
That 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 was of a certain nature. The specific nature of the consonants that trigger back umlaut or block it varied from dialect to dialect.
The long high vowels of Middle High German underwent breaking during the transition to Early New High German: /i: y: u:/ -> /a a/. In Yiddish, the diphthongization affected the long mid vowels as well: /?: o: ø: i: y: u:/ -> / a a /
This change started as early as the 12th century in Upper Bavarian and reached Moselle Franconian only in the 16th century. It did not affect Alemannic or Ripuarian dialects, which still retain the original long vowels.
Vowel breaking is present in Scottish Gaelic with the following changes occurring often but variably between dialects: Archaic Irish e: -> Scottish Gaelic i? and Archaic Irish o: -> Scottish Gaelic u?  Specifically, central dialects have more vowel breaking than others.
Many Romance languages underwent vowel breaking. The Vulgar Latin open vowels e /?/ and o /?/ 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 /?/, as it did not have /?/:
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 vowel-breaking before an original laryngeal in Greek, Armenian and Tocharian but that the other Indo-European languages kept the monophthongs:
However, the hypothesis has not been widely adopted.