In a diphthongal manner; in the way that relates to or involves diphthongs.
From 'diphthongal' + '-ly' (in the manner of). This adverbial form extends the adjectival term to describe how something is done or occurs with diphthongal characteristics.
When linguists describe how certain vowels are pronounced 'diphthongally,' they're noting that speakers are blending two vowel qualities together rather than holding a single pure sound.
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