Diphthongization

In historical linguistics, vowel breaking, vowel fracture, or diphthongization is the change of a monophthong into a diphthong or triphthong.

Vowel breaking may be unconditioned or conditioned. It may be triggered by the presence of another sound or by stress, or it may 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.

Vowel breaking is sometimes not assimilatory, not triggered by a neighboring sound. This is the case with the Great Vowel Shift in English: all cases of /iː uː/ changed to diphthongs.

Sometimes vowel breaking occurs only in stressed syllables. For instance, Vulgar Latin open-mid /ɛ 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].

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