In phonetics, vowel roundedness refers to the amount of rounding in the lips during the articulation of a vowel. That is, it is vocalic labialization. When pronouncing a rounded vowel, the lips form a circular opening, while unrounded vowels (also called spread vowels) are pronounced with the lips relaxed. In most languages, front vowels tend to be unrounded, while back vowels tend to be rounded. But some languages, such as French and German, distinguish rounded and unrounded front vowels of the same height, while Vietnamese distinguishes rounded and unrounded back vowels of the same height. Gahuku has all unrounded vowels.
In the International Phonetic Alphabet vowel chart, rounded vowels are the ones that appear on the right in each pair of vowels. There are also diacritics, ⟨ ̹ ⟩ (U+0339 ̹ combining right half ring below) and ⟨ ̜ ⟩ (U+031C ̜ combining left half ring below), to indicate greater and lesser degrees of rounding, respectively. These 'more' and 'less rounded' diacritics are sometimes also used with consonants to indicate degrees of labialization.
Types of rounding 
There are two types of vowel rounding: protruded and compressed. In protruded rounding, the corners of the mouth are drawn together and the lips protrude like a tube, with their inner surface visible. In compressed rounding, the corners of the mouth are drawn together, but the lips are also drawn together horizontally ("compressed") and do not protrude, with only their outer surface visible. That is, in protruded vowels the inner surfaces of the lips form the opening (thus the alternate term endolabial), while in compressed vowels it is the margins of the lips which form the opening (thus exolabial). Catford (1982:172) observes that back and central rounded vowels, such as German /o/ and /u/, are typically protruded, while front rounded vowels such as German /ø/ and /y/ are typically compressed. Back or central compressed vowels and front protruded vowels are uncommon, and a contrast between the two types has been found to be phonemic in only one instance. There are no dedicated IPA diacritics to represent the distinction.
The distinction between protruded [u] and compressed [y] holds for the semivowels [w] and [ɥ] as well as labialization. In Akan, for example, the [ɥ] is compressed, as are labio-palatalized consonants as in Twi [tɕᶣi̘] "Twi" and adwuma [adʑᶣu̘ma] "work", whereas [w] and simply labialized consonants are protruded. In Japanese, the /w/ is compressed rather than protruded, paralleling the Japanese /u/. The distinction applies marginally to other consonants. In Southern Teke, the sole language reported to have a phonemic /ɱ/, the labiodental sound is "accompanied by strong protrusion of both lips", whereas the [ɱ] found as an allophone of /m/ before /f, v/ in languages such as English is not protruded, as the lip contacts the teeth along its upper or outer edge. And in at least one account of speech acquisition, a child's pronunciation of clown involves a lateral [f] with the upper teeth contacting the upper-outer edge of the lip, but in crown a non-lateral [f] is pronounced with the teeth contacting the inner surface of the protruded lower lip.
Some vowels transcribed with rounded IPA letters may not be rounded at all. An example is /ɒ/, which in English has very little if any rounding of the lips. The "throaty" sound of English /ɒ/ is instead accomplished with sulcalization, a furrowing of the back of the tongue also found in non-rhotic /ɜː/ (Lass 1984:124).
It is possible to mimic the acoustic effect of rounded vowels by narrowing the cheeks, so-called "cheek rounding", which is inherent in back protruded (but not front compressed) vowels. This technique is used by ventriloquists to mask the visible rounding of back vowels like [u]. It is not clear if it is employed by languages with rounded vowels that do not utilize visible rounding.
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 vowel /ɔ/ is pronounced [u̯ɔ] after labial consonants, an allophonic effect salient enough to be encoded in pinyin transliteration: alvelar /tu̯ɔ/ (Pinyin duo) 'many' vs. labial /pu̯ɔ/ (Pinyin bo) 'wave'. In Vietnamese, the opposite assimilation takes place: velar codas /k/ and /ŋ/ are pronounced as labialized [kʷ] and [ŋʷ], or even labial-velar [kp] and [ŋm], 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 [ku] and [ko] are phonemically /kwɨ/ and /kwə/; similarly, Ubykh [ku] and [ko] are phonemically /kʷə/ and /kʷa/. A few ancient Indo-European languages are also noted for having labiovelar consonants, including Latin.
See also 
- Close back compressed vowel
- Close central compressed vowel
- Close front compressed vowel
- List of phonetics topics
- The International Phonetic Alphabet in Unicode, UCL Division of Psychology & Language Sciences
- Endolabial: also called protruded, lip-pouting, horizontal lip-rounding, outrounding, and inner rounding (Trask 1996:180)
Exolabial: also called compressed, pursed, vertical lip-rounding, inrounding, or outer rounding (Trask 1996:252)
Henry Sweet noted in 1890 that "the term 'inner rounding' derives from the use of the inner surfaces of the lips; the synonymous 'outrounding' derives from the forward projection of the lips. Both terms are justifiable, but their coexistence is likely to lead to serious confusion." (Trask 1996:180.)
- Henry Sweet (A handbook of phonetics, 1877) noted that they are less distinctive from unrounded vowels than their counterparts.
- Japanese has a back compressed rather than protruded vowel u (Okada 1999:118); Swedish has a back compressed vowel ⟨o⟩ and both a front compressed ⟨u⟩ and a front protruded vowel ⟨y⟩ (Engstrand 1999:141); the front rounded vowels contrast in ryta 'roar' and ruta 'window pane' (Ladefoged & Maddieson 1996:295).
- The Akan (Twi-Fante) Language: Its Sound Systems and Tonal Structure. Florence Abena Dolphyne, Ghana Universities Press, Accra, 1988. ISBN 9964-3-0159-6
- Christiane Paulian (1975) Le kukuya, langue teke du Congo: phonologie, classes nominales
- Kelly & Local (1989) Doing phonology, p 41
- Henry Sweet, 1877, A handbook of phonetics, vol. 2, pp 14, 20
- W. Sidney Allen (1978) Vox Latina
- Catford, J. C. (1982) Fundamental Problems in Phonetics. Bloomington: Indiana University Press.
- Engstrand, O. (1999). Illustrations of the IPA: Swedish in IPA (1999).
- IPA – The International Phonetic Association (1999). Handbook of the International Phonetic Association. Cambridge: Cambridge University Press.
- Ladefoged, P. & Maddieson, I. (1996). The Sounds of the World's Languages. Oxford: Blackwell. ISBN 0-631-19814-8.
- Okada, H. (1999). Illustrations of the IPA: Japanese in IPA (1999).
- Trask, R. L. (1996). A dictionary of phonetics and phonology. Routledge: London & New York.
- The dictionary definition of endolabial at Wiktionary
- The dictionary definition of exolabial at Wiktionary
- The dictionary definition of unrounded at Wiktionary