In phonetics, rounding refers to the strength of the rounding of the lips during the articulation of a vowel. That's a vowel Labialization. The opposite of rounding is the spread. If a rounded vowel is pronounced, the lips form a circular opening, while unrounded vowels are pronounced with relaxed lips.
In most languages, vowels tend forward to being apart, and back to being rounded. But some languages such as German and French distinguish rounded front vowels and spread the same vowel height ( eg, [ ɛ ] and [œ ] in and be able to ) and Vietnamese distinguishes between rounded and spread hind vowels.
Rounded consonants are called labialized.
In the vowel chart of the International Phonetic Alphabet (IPA ), the rounded vowels on the right side of each pair. There are also two diacritics to indicate a stronger or weaker rounding, here for example [ ɔ̹ ] and [ ɔ̜ ] the sound [ ɔ ]. The diacritics are sometimes also used in consonants for the degree of Labialization. For example, in the Athabaskan language Hupa velar fricatives with voiceless between three levels of Labialization distinction is transcribed as either [x x x ʷ ] or [x x x ʷ ʷ ]. The extended IPA has two additional symbols for the degree of rounding: [ ə ͍ ] and [ ʒœ ].