Labialization is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally restricted to consonants. When vowels involve the lips, they are called rounded.
The most common labialized consonants are labialized velars. Most other labialized sounds also have simultaneous velarization, and the process may then be more precisely called labio-velarization.
American English has three degrees of labialization: tight rounded (/w/, initial /r/), slight rounded (/ʃ/, /ʒ/, /tʃ/, /dʒ/, coloring /r/), and unrounded, which in vowels is sometimes called 'spread'. These secondary articulations are not universal. For example, French shares the English slight rounding of /ʃ/, /ʒ/ while Russian does not have slight rounding in its postalveolar fricatives (/ʂ ʐ ɕ ʑ/).
A few languages, including Arrernte and Mba, have contrastive labialized forms for almost all of their consonants.
Out of 706 language inventories surveyed by Ruhlen (1976), labialization occurred most often with velar (42%) and uvular (15%) segments and least often with dental and alveolar segments. With non-dorsal consonants, labialization may include velarization as well. Labialization is not restricted to lip-rounding. The following articulations have either been described as labialization, or been found as allophonic realizations of prototypical labialization:
In North America, languages from a number of families have sounds that sound labialized (and vowels that sound rounded) without participation of the lips. See Tillamook language for an example.
In the International Phonetic Alphabet, labialization of velar consonants is indicated with a raised w modifier [ʷ] (Unicode U+02B7), as in /kʷ/. (Elsewhere this diacritic generally indicates simultaneous labialization and velarization.) There are also diacritics, respectively [ɔ̹], [ɔ̜], to indicate greater or lesser degrees of rounding. These are normally used with vowels, but may occur with consonants. For example, in the Athabaskan languageHupa, voiceless velar fricatives distinguish three degrees of labialization, transcribed either /x/, /x̹/, /xʷ/ or /x/, /x̜ʷ/, /xʷ/.
If precision is desired, the Abkhaz and Ubykh articulations may be transcribed with the appropriate fricative or trill raised as a diacritic: [tᵛ], [tᵝ], [tʙ], [tᵖ].
For simple labialization, Ladefoged & Maddieson (1996) resurrected an old IPA symbol, [ ̫], which would be placed above a letter with a descender such as ɡ. However, their chief example is Shona sv and zv, which they transcribe /s̫/ and /z̫/ but which actually seem to be whistled sibilants, without necessarily being labialized. Another possibility is to use the IPA diacritic for rounding, distinguishing for example the labialization in English soon[s̹] and [sʷ]swoon. The open rounding of English /ʃ/ is also unvelarized.
Labialization also refers to a specific type of assimilatory process where a given sound become labialized due to the influence of neighboring labial sounds. For example, /k/ may become /kʷ/ in the environment of /o/, or /a/ may become /o/ in the environment of /p/ or /kʷ/.
In the Northwest Caucasian languages as well as some Australian languages rounding has shifted from the vowels to the consonants, producing a wide range of labialized consonants and leaving in some cases only two phonemic vowels. This appears to have been the case in Ubykh and Eastern Arrernte, for example. The labial vowel sounds usually still remain, but only as allophones next to the now-labial consonant sounds.
^As a mnemonic, the more-rounded diacritics resembles the rounded vowel ⟨ɔ⟩.
^International Phonetic Association (1999). Handbook of the International Phonetic Association: A Guide to the Use of the International Phonetic Alphabet. Cambridge University Press. p. 190. ISBN978-0-52163751-0.