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.
In phonology, labialization may also refer to a type of assimilation process.
Labialization is the most widespread secondary articulation in the world's languages. It is phonemically contrastive in Northwest Caucasian (e.g. Adyghe), Athabaskan, and Salishan language families, among others. This contrast is reconstructed also for Proto-Indo-European, the common ancestor of the Indo-European languages; and it survives in Latin and some Romance languages.
American English has three degrees of labialization: tight rounded (/w/, initial /r/), slight rounded (/?/, /?/, /t?/, /d?/, 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 /?/, /?/ while Russian does not have slight rounding in its postalveolar fricatives (/? ? ? ?/).
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:
- Labiodental frication, found in Abkhaz
- Complete bilabial closure, [d?b, t?p, t?p'], found in Abkhaz and Ubykh
- "Labialization" (/w/, //, and /k?/) without noticeable rounding (protrusion) of the lips, found in the Iroquoian languages. It may be that they are compressed.
- Rounding without velarization, found in Shona and in the Bzyb dialect of Abkhaz.
Eastern Arrernte has labialization at all places and manners of articulation; this derives historically from adjacent rounded vowels, as is also the case of the Northwest Caucasian languages. Marshallese also has phonemic labialization as a secondary articulation at all places of articulation except for labial consonants and coronal obstruents.
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 [?] (Unicode U+02B7), as in /k?/. (Elsewhere this diacritic generally indicates simultaneous labialization and velarization. ) There are also diacritics, respectively , , to indicate greater or lesser degrees of rounding. These are normally used with vowels, but may occur with consonants. For example, in the Athabaskan language Hupa, voiceless velar fricatives distinguish three degrees of labialization, transcribed either /x/, /x?/, /x?/ or /x/, /x/, /x?/.
The extensions to the IPA has two additional symbols for degrees of rounding: Spread  and open-rounded  (as in English). It also has a symbol for labiodentalized sounds, [t?].
If precision is desired, the Abkhaz and Ubykh articulations may be transcribed with the appropriate fricative or trill raised as a diacritic: [t?], [t?], [t?], [t?].
For simple labialization, Ladefoged & Maddieson (1996) resurrected an old IPA symbol, [ ?], which would be placed above a letter with a descender such as ?. However, their chief example is Shona sv and zv, which they transcribe /s?/ and /z?/ 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 [s?] and [s?] swoon. The open rounding of English /?/ 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 /k?/ in the environment of /o/, or /a/ may become /o/ in the environment of /p/ or /k?/.
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.
- labialized voiceless bilabial stop ( (in Chaha, Paha)
- labialized voiced bilabial stop ( (in Chaha, Paha)
- labialized voiceless alveolar stop (in Archi, Abkhaz, Lao, Paha, Ubykh)
- labialized voiced alveolar stop (in Archi, Abkhaz, Ubykh)
- labialized voiceless velar stop (in Abaza, Abkhaz, Adyghe, Halkomelem, Kabardian, Taos, Chipewyan, Hadza, Gwich'in, Tlingit, Akan, Nez Perce, Archi, Cantonese, Wari', Chaha, Dahalo, Hausa, Igbo, Italian, Lao, Latin, Nahuatl, Nawat, Ossetic, Paha, Portuguese, Thai, Tigrinya, Hiw, Ubykh, Bearlake Slavey, Breton)
- labialized voiced velar stop ( (in Abaza, Abkhaz, Adyghe, Akan, Archi, Chaha, Dahalo, Hausa, Oowekyala, Ossetic, Hadza, Igbo, Gwich'in, Kabardian, Paha, Portuguese, Tigrinya, Ubykh, Breton)
- labialized voiceless uvular stop ( (in Abaza, Abkhaz, Adyghe, Kabardian, Ossetic, Paha, Tlingit, Nez Perce, Ubykh)
- labialized pharyngealized voiceless uvular stop [q] (in Archi, Ubykh)
- labialized voiced uvular stop ( (in Oowekyala, Kwak'wala, Tsakhur)
- labialized glottal stop ( (in Adyghe, Kabardian, Lao, Tlingit)
- labialized prenasalized voiced bilabial plosive [?b?] (in Tamambo)
- Sibilant affricates
- labialized voiceless alveolar affricate (in Adyghe, Archi, Lezgian, Tsakhur)
- labialized voiced alveolar affricate (in Adyghe, Dahalo)
- labialized voiceless palato-alveolar affricate (in Archi, Abaza, Adyghe, Paha, Aghul, German)
- labialized voiced palato-alveolar affricate (in Abaza, Aghul, Tsakhur, German)
- labialized voiceless alveolo-palatal affricate [t] (in Abkhaz, Akan, Ubykh)
- labialized voiced alveolo-palatal affricate [d] (in Abkhaz, Akan, Ubykh)
- labialized voiceless alveolar sibilant (in Archi, Lao, Lezgian)
- labialized voiced alveolar sibilant (in Archi, Tsakhur, Lezgian)
- labialized voiceless palato-alveolar sibilant (in Archi, Abaza, Abkhaz, Adyghe, Paha, Aghul, Ubykh)
- labialized voiced palato-alveolar sibilant (in Archi, Abaza, Abkhaz, Adyghe, Aghul, Ubykh)
- labialized voiceless retroflex sibilant (in Bzhedug)
- labialized voiced retroflex sibilant (in Bzhedug)
- labialized voiceless alveolo-palatal sibilant (in Abkhaz, Ubykh)
- labialized voiced alveolo-palatal sibilant (in Abkhaz, Ubykh)
Central non-sibilant fricatives
- labialized voiceless bilabial fricative 
- labialized voiced bilabial fricative  (in Tamambo)
- labialized voiceless labiodental fricative (in Hadza, Chaha)
- labialized voiced labiodental fricative )
- labialized voiceless dental fricative (in Paha)
- labialized voiced dental fricative (in Paha)
- labialized voiceless palatal fricative (in Akan)
- labialized voiceless velar fricative (in Abaza, Adyghe, Avestan, Chaha, Halkomelem, Kabardian, Oowekyala, Taos, Navajo, Tigrinya, Lillooet, Tlingit)
- labialized voiced velar fricative (in Abaza, Navajo, Lillooet, Gwich'in, possibly Proto-Indo-European)
- labialized voiceless uvular fricative (in Abkhaz, Adyghe, Archi, Halkomelem, Kabardian, Lillooet, Tlingit, Wari', Chipewyan, Oowekyala, Ossetic, Ubykh)
- labialized pharyngealized voiceless uvular fricative  (in Abkhaz, Archi, Ubykh)
- labialized voiced uvular fricative (in Abkhaz, Adyghe, Chipewyan, Kabardian, Ubykh)
- labialized pharyngealized voiced uvular fricative  (in Archi, Ubykh)
- labialized voiceless pharyngeal fricative (in Abaza, Abkhaz)
- labialized voiced pharyngeal fricative (in Abaza, Lillooet)
- labialized bilabial ejective (In Adyghe)
- labialized alveolar ejective (in Abkhaz, Adyghe, Ubykh)
- labialized velar ejective (in Abaza, Abkhaz, Adyghe, Archi, Bearlake Slavey, Halkomelem, Kabardian, Ossetic, Tlingit, Ubykh)
- labialized palato-alveolar ejective fricative (in Adyghe)
- labialized uvular ejective (in Abaza, Abkhaz, Archi, Halkomelem, Hakuchi, Tlingit, Ubykh)
- labialized pharyngealized uvular ejective [q'] (in Archi, Ubykh)
- labialized alveolar ejective affricate [t?s?'] (in Archi, Khwarshi)
- labialized alveolar lateral ejective affricate [t'] (in Khwarshi)
- labialized palato-alveolar ejective affricate [t'] (in Abaza, Archi, Khwarshi)
- labialized alveolo-palatal ejective affricate [t'] (in Abkhaz, Ubykh)
- labialized retroflex ejective affricate [?'] (allophonic in Adyghe)
- labialized velar lateral ejective affricate [k'] (in Archi)
- labialized velar ejective fricative [x?'] (in Tlingit)
- labialized uvular ejective fricative ['] (in Tlingit)
- ^ Ladefoged & Maddieson (1996:148)
- ^ 
- ^ 
- ^ As a mnemonic, the more-rounded diacritics resembles the rounded vowel ⟨?⟩.
- ^ 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. ISBN 978-0-52163751-0.
- ^ See . Archived May 16, 2008, at the Wayback Machine
- ^ John Esling (2010) "Phonetic Notation", in Hardcastle, Laver & Gibbon (eds) The Handbook of Phonetic Sciences, 2nd ed.
- ^ Yanushevskaya & Bun?i? (2015:223) harvcoltxt error: no target: CITEREFYanushevskayaBun?i?2015 (help)