Roundedness and Labialization
(Protrusion) roundedness is the vocalic equivalent of consonantal labialization. As such, rounded vowels and labialized consonants affect each other through phonetic assimilation: Rounded vowels labialize consonants, and labialized consonants round vowels.
In many languages such effects are minor phonetic detail, but in some cases they become significant. For example, in Mandarin Chinese, the unrounded diphthong /ɯ̯ʌ/ is pronounced after labial consonants, an allophonic effect salient enough to be encoded in pinyin transliteration: velar /xɯ̯ʌ/ he vs. labial /pu̯ɔ/ bo. In Vietnamese, the opposite assimilation takes place: velar codas /k/ and /ŋ/ are pronounced as labialized and, or even labial-velar and, after the rounded vowels /u/ and /o/. In the Northwest Caucasian languages of the Caucasus and the Sepik languages of Papua New Guinea, historically rounded vowels have become unrounded, with the rounding being taken up by the consonant, so that, for example, Sepik and are phonemically /kwɨ/ and /kwə/; similarly, Ubykh and are phonemically /kʷə/ and /kʷa/.
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