diphthongize
Verb
Meaning
To change into a diphthong, as by affixing another vowel to a simple vowel.
Source: Webster's dictionarySynonyms
Hypernyms
Hyponyms
Examples
This vowel diphthongized in Germanic
Limburgish is typically a two-tone system, distinguishing between level high and falling, but the tones can be realized in other ways depending on syntax, and some vowels diphthongize or monophthongize under certain tones.
However, there are recognized situations in which other vowel pairs diphthongize.
There is also a tendency to diphthongize simple vowels, e.g., ū becomes eo or eu.