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.