Labialisation 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 used to refer to consonants. When vowels involve the lips, they are usually called rounded.
Labialisation may also refer to a type of assimilation process.
Labialisation is the most widespread secondary articulation in the world's languages. It is phonemically contrastive in the Northwest Caucasian, Athabaskan, and Salishan language families, among others. This contrast is reconstructed also for Proto-Indo-European, the common ancestor of the Indo-European languages.
American English has three degrees of (phonetic) labialization: Fully rounded /w/ and initial /ɹ/, open-rounded , and unrounded, in which vowels are sometimes called spread. These secondary articulations are not universal. For example, while French shares the English open-rounding of while Russian does not have rounding of its postalveolar fricatives ().
Out of 706 language inventories surveyed by , labialisation occurred most often with velar (42%) and uvular (15%) segments and least often with dental and alveolar segments. With non-dorsal consonants, labialisation may include velarisation as well. Labialisation is not restricted to lip-rounding. The following articulations have either been described as labialisation, or been found as allophonic realisations of prototypical labialisation:
Eastern Arrernte is a language with labialisation at all places and manners of articulation. The labialisation derives historically from adjacent rounded vowels, as is also the case of the Northwest Caucasian languages.
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, resurrected an old IPA symbol, [ ̫]. 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. The open rounding of English /ʃ/ is also unvelarized.
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.