British spelling: having been converted into or pronounced as a diphthong; having undergone diphthongisation.
From 'diphthongise' (British verb) + '-d' (past tense). This is the British past tense and adjectival form describing vowels that have transformed into diphthongal sounds.
When British linguists write that a vowel has been 'diphthongised,' they're documenting how English pronunciation has shifted—it's the same sound change, just spelled the British way!
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