Labialized velar consonant
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A labialized velar is a velar consonant which is labialized, that is, which has an /w/-like secondary articulation. Common examples are [kʷ, ɡʷ, xʷ, ŋʷ], which are pronounced like a [k, ɡ, x, ŋ] with rounded lips. See for example the labialized voiceless velar plosive [kʷ]. Such sounds occur across Africa, throughout the Americas, in the Caucasus, etc.
Labialized velar approximants 
The most common labiovelar consonant is the voiced approximant [w]. This is normally a labialized velar, as is its vocalic cousin [u]. (Labialization is called rounding in vowels, and a velar place is called back.) However, languages such as Japanese and perhaps the Northern Iroquoian languages have something closer to a true labial–velar approximant, where the lips come together. In close transcription, the symbol [w] may be avoided in such cases, or it may be used with an under-rounding diacritic, as [w̜].
[w] and its voiceless equivalent are the only labialized velars with dedicated IPA symbols:
|voiceless labialized velar approximant||English||which||[ʍɪtʃ]1||'which'|
|voiced labialized velar approximant||English||witch||[wɪtʃ]||'witch'|
1In dialects that distinguish between which and witch.
The voiceless approximant is traditionally called a "voiceless labial–velar fricative", but true doubly articulated fricatives are not known to be used in any language, as they are quite difficult to pronounce and even more to aurally distinguish. (However, very occasionally the symbol [ʍ] is used for a labialized velar fricative, [xʷ]. This usage is not approved by the IPA.)
Historical development 
Labialized velars frequently derive from a plain velar followed by a rounded (labialized) vowel such as [u] or [o]. In turn, they may sometimes develop into simple bilabial consonants. An example of this is the development of Proto-Indo-European *kʷ, *gʷ before the vowel *a or *o into Greek /p, b/, producing unobvious cognates such as English come and Greek-derived basis. The full sequence is demonstrated by the Satsuma dialect of Japanese: in northern Satsuma, standard Japanese [kue] 'eat!' has contracted to [kʷe]; in southern Satsuma, it has proceeded further to [pe].