British spelling: to change a vowel sound into a diphthong, or to pronounce as a diphthong.
From 'diphthong' + '-ise' (British verb suffix). The British convention uses '-ise' while American English uses '-ize,' both derived from Greek '-izein' meaning 'to make or become.'
This is the British preference for 'diphthongise' over American 'diphthongize'—a perfect example of how even scientific terminology splits between English-speaking countries!
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