diphthongize

• diphthongise (non-Oxford British English)

From diphthong +‎ -ize.

• (General American) IPA^(key): /ˈdɪfθɑŋaɪz/, /ˈdɪp-/
• (Received Pronunciation) IPA^(key): /ˈdɪfθɒŋaɪz/, /ˈdɪp-/

diphthongize (third-person singular simple present diphthongizes, present participle diphthongizing, simple past and past participle diphthongized)

1. (transitive, phonetics) To change to a diphthong, as by inserting or removing a vowel.

   Coordinate term: monophthongize

   • 2022, Daisy L. Neijman, Icelandic: An Essential Grammar, Routledge, →ISBN, page 5:

     Some of these [monophthongs] will be diphthongised under the influence of following sounds.

2. (intransitive, phonetics) To become a diphthong.