In phonetics, vowel roundedness refers to the amount of rounding in the lips during the articulation of a vowel. It is labialization of a vowel. When a rounded vowel is pronounced, the lips form a circular opening, and unrounded vowels are pronounced with the lips relaxed. In most languages, front vowels tend to be unrounded, and back vowels tend to be rounded. However, some languages, such as French, German and Icelandic, distinguish rounded and unrounded front vowels of the same height (degree of openness), and Vietnamese distinguishes rounded and unrounded back vowels of the same height. Alekano has only 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, ̹ COMBINING RIGHT HALF RING BELOW and ̜ COMBINING LEFT HALF RING BELOW, to indicate greater and lesser degrees of rounding, respectively. Thus [o?] has less rounding than cardinal [o], and [o?] has more (closer to the rounding of cardinal [u]). These diacritics can also be used with unrounded vowels:  is more spread (less rounded) than cardinal [?], and  is less spread than cardinal [?].
There are two types of vowel rounding: protrusion and compression. 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), whereas in compressed vowels it is the margins of the lips which form the opening (thus exolabial). Catford (1982, p. 172) observes that back and central rounded vowels, such as German and , are typically protruded, whereas front rounded vowels such as German and 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, but the superscript IPA letter ⟨⟩ or ⟨⟩ can be used for compression and ⟨⟩ for protrusion. Compressed vowels may be pronounced either with the corners of the mouth drawn in, by some definitions rounded, or with the corners spread and, by the same definitions, unrounded. The distinction may be transcribed ⟨?? u?⟩ vs ⟨?? ⟩ (or ⟨ u?⟩ vs ⟨ ⟩).
The distinction between protruded [u] and compressed [y] holds for the semivowels [w] and [?] as well as labialization. In Akan, for example, the is compressed, as are labio-palatalized consonants as in Twi [ti?] "Twi" and adwuma [adu?ma] "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 , the labiodental sound is "accompanied by strong protrusion of both lips", whereas the 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. Also, 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 , the vowel of lot, which in Received Pronunciation has very little if any rounding of the lips. The "throaty" sound of the vowel is instead accomplished with sulcalization, a furrowing of the back of the tongue also found in , the vowel of nurse.
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. The technique is used by ventriloquists to mask the visible rounding of back vowels like [u]. It is not clear if it is used by languages with rounded vowels that do not use visible rounding.
|Semivowel||j ?||j? ?||? /w? w|
|Close||i y y?||? ÿ ?||? /u? u|
|Near-close||? ?||/ ?|
|Close-mid||e ø ø?||? ø? ?||? /o? o|
|Open-mid||? oe oe?||? oe? ?||? / ?|
Of the open-mid vowels, [oe?] occurs in Swedish and Norwegian. Central [oe?] and back  have not been reported to occur in any language.
The lip position of unrounded vowels may be classified into two groups: spread and neutral. Front vowels are usually pronounced with the lips spread, and the spreading becomes more significant as the height of the vowel increases. Open vowels are often neutral, i.e. neither rounded nor spread, because the open jaw allows for limited rounding or spreading of the lips. This is reflected in the IPA's definition of the cardinal , which is unrounded yet not spread either.
Protruded rounding is the vocalic equivalent of consonantal labialization. Thus, rounded vowels and labialized consonants affect one another by phonetic assimilation: Rounded vowels labialize consonants, and labialized consonants round vowels.
In many languages, such effects are minor phonetic detail, but in others, they become significant. For example, in Standard Chinese, the vowel /?/ is pronounced [u] after labial consonants, an allophonic effect that is so important that it is encoded in pinyin transliteration: alveolar /tu/ [twó] (?; ) 'many' vs. labial /pu/ [pwó] (?; ) 'wave'. In Vietnamese, the opposite assimilation takes place: velar codas /k/ and /?/ are pronounced as labialized [k?] and  or even labial-velar [kp] and [?m], 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. Thus, Sepik [ku] and [ko] are phonemically /kw?/ and /kw?/.
It is rare for accents of English to differentiate vowels only by their roundedness. Minimal pairs like this can be found in some British dialects (such as the Cardiff dialect, Geordie and Port Talbot English) as well as in General South African English. Most commonly, they involve a contrastive pair of close-mid vowels, with the unrounded vowel being either SQUARE or a monophthongal FACE and the rounded counterpart being NURSE . Contrasts based on roundedness are rarely categorical in English and they may be enhanced by additional differences in height, backness or diphthongization.
|Cardiff||[ei]||SQUARE may be open-mid .|
|Geordie||FACE may be diphthongal [ ~ e?], whereas|
NURSE may be back or unrounded [?: ~ ?:].
|Port Talbot||The accent does not feature the pane-pain merger.|
General South African English is unique among accents of English in that it can feature up to three front rounded vowels, with two of them having unrounded counterparts.
|Height||Unr. vowel||Rnd. vowel||Notes|
|lexical set||realization||lexical set||realization|
|Close||FLEECE||GOOSE||GOOSE may be central .|
|Open-mid||(unpaired)||GOAT||GOAT may be diphthongal [oe].|
The potential contrast between the close-mid and the open-mid is hard to perceive by outsiders, making utterances such as the total onslaught [ð? 'toe:tl? '?nslo:t] sound almost like the turtle onslaught [ð? 'tø:tl? '?nslo:t].