Having undergone diphthongization; pronounced as or containing a diphthong.
From 'diphthong' + '-ed' (having the quality of). This adjective describes sounds or vowels that have developed or been transformed into diphthongal forms.
When you see a vowel marked as 'diphthonged' in a historical description, it's linguists' way of tracking how languages have transformed their sound systems over centuries of change.
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