A labialized velar is a velar consonant that is labialized, with a /w/-like secondary articulation. Common examples are [kʷ, ɡʷ, xʷ, ŋʷ], which are pronounced like a [k, ɡ, x, ŋ], with rounded lips, such as the labialized voiceless velar plosive [kʷ]. Such sounds occur across Africa and the Americas, in the Caucasus, etc.
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Labialized velar approximants
The most common labiovelar consonant is the voiced approximant [w]. It 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 in which the lips come together. In close transcription, the symbol [w] may be then avoided or used with an under-rounding diacritic: [w̜].
[w] and its voiceless equivalent are the only labialized velars with dedicated IPA symbols:
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 difficult to distinguish. (The symbol [ʍ] is very occasionally used for a labialized velar fricative, [xʷ] even if that 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 *a or *o into Greek /p, b/, producing cognates as different as English come and 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].
A notable development is the initial *kʷ in Proto-Indo-European interrogative words. In English, it developed into wh or h (how), pronounced /w/ in most dialects and /h/, respectively, via Grimm's law followed by wh-cluster reductions. By contrast, in Latin and its descendents, the Romance languages, that developed into qu (later Spanish cu (cuando) and c (como)), pronounced variously as /kw/ or /k/. See etymology of English interrogative words for details. The English phonemic spelling kw for qu (as in kwik) echoes its origin.