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. Occurrence 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. It is also found in the Cushitic and Ethio-Semitic languages. American English labializes to various degrees. A few languages, including Arrernte and Mba, have contrastive labialized forms for almost all of their consonants. Types Out of 706 language inventories surveyed by , 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, , found in Abkhaz and Ubykh "Labialization" (, , and ) 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. Transcription In the International Phonetic Alphabet, labialization of velar consonants is indicated with a raised w modifier (Unicode U+02B7), as in . (Elsewhere this diacritic generally indicates simultaneous labialization and velarization.) There are also diacritics, respectively , to indicate greater or lesser degrees of rounding.