fù hé yuán yīn
diphthong (such as putonghua ɑi, uei etc)
yuán yīn hé xié
vowel harmony (in phonetics of Altaic languages)
juǎn shé yuán yīn
retroflex vowel (e.g. the final r of putonghua)
juǎn shé yuán yīn
retroflex vowel (e.g. the final r of putonghua)