The process or result of changing a simple vowel into a diphthong, or the act of becoming diphthongal.
From 'diphthongal' + '-ation' (process/result). This noun form describes the linguistic phenomenon where pure vowels gradually develop gliding characteristics over generations of speakers.
The Great Vowel Shift in English (around 1350-1700) involved massive diphthongation—what used to be simple vowels became the gliding sounds we hear today in words like 'name' and 'stone'!
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