|Sound change and alternation|
In historical linguistics, vowel breaking (sometimes called vowel fracture) is the change of a monophthong into a diphthong or triphthong. The change into a diphthong is also known as diphthongization. Vowel breaking is often distinguished from diphthongization and defined more narrowly as a harmonic (i.e., assimilatory) process involving diphthongization triggered by a following vowel or consonant. The original pure vowel typically breaks into two segments, where 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 (e.g., velar or pharyngeal), and the second segment may be /i/ (a front vowel) if the following vowel or consonant is front (e.g., 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.
Southern American English 
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].
Middle English 
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.
Old English 
Old English breaking is a process in prehistoric Old English whereby stressed short and long i, e, æ become short and long diphthongs spelled io, eo, ea (respectively), when followed by h or by r, l + another consonant (short vowels only), and sometimes w (only for certain short vowels).
- PG *fallan → feallan "fall"
- PG *erþō → eorþe "earth"
Back umlaut is a process in late prehistoric Old English whereby short i, e, æ become short diphthongs spelled io, eo, ea (respectively) 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.
Old Norse 
- PG *ek(a) "I" → (east) ON jak, Swedish jag, Danish and Norwegian Bokmål jeg (but Jutlandic æ, a, Icelandic ek → ég, Nynorsk eg)
- PG *hertōn "heart" → ON hjarta, Swedish hjärta, Faroese hjarta, Danish hjerte
- PG *erþō "earth" → Proto-Norse *erþū → ON jǫrð, Swedish, Danish jord, Faroese jørð
Scottish Gaelic 
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.
Romance languages 
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. The result of breaking varies between languages: e and o became ie and ue in Spanish, ie and uo in Italian, but ea and oa in Romanian. Thus:
|Open||petra, focus||piedra, fuego||pierre, feu||pietra, fuoco||pedra, fogo|
|Closed||festa, porta||fiesta, puerta||fête, porte||festa, porta||festa, porta|
Highlighted words underwent vowel breaking.
Some scholars believe that Proto-Indo-European (PIE) i, u has a kind of breaking before an original laryngeal in Greek, Armenian and Tocharian, whereas the other Indo-European languages have monophthongs. Typical examples are:
- PIE *gʷih3wos → *gʷioHwos "alive" → Gk. ζωός, Toch. B śāw-, śāy- (but Skt. jīvá-, Lat. vīvus)
- PIE *protih3kʷom → *protioHkʷom "front side" → Gk. πρόσωπον "face", Toch. B pratsāko "breast" (but Skt. prátīka-)
- PIE *duh2ros → *duaHros "long" → Gk. δηρός, Arm. *twār → erkar (Skt. dūrá-, Lat. dūrus).
However, the hypothesis is not adopted by most handbooks.
See also 
- Crowley, Terry. (1997) An Introduction to Historical Linguistics. 3rd edition. Oxford University Press.
- The American Heritage Dictionary of the English Language, Fourth Edition 2000 by Houghton Mifflin Company.
- Robert B. Howell 1991. Old English breaking and its Germanic analogues (Linguistische Arbeiten, 253.). Tübingen: Max Niemeyer
- J. Svensson, Diftongering med palatalt förslag i de nordiska språken, Lund 1944.
- H. Paul, "Zur Geschichte des germanischen Vocalismus", Beiträge zur Geschichte der deutschen Sprache und Kultur 6 (1879) 16-30.
- K. M. Nielsen, Acta Philologica Scandinavica 24 (1957) 33-45.
- The Celtic Languages. p. 152.
- F. Normier, in: Zeitschrift für vergleichende Sprachforschung 91 (1977) 171-218; J.S. Klein, in: Die Laryngaltheorie und die Rekonstruktion des indogermanischen Laut- und Formensystems, Heidelberg 1988, 257-279; J.E. Rasmussen, in: Selected Papers on Indo-European Linguistics, Copenhagen 1999, 442-458.