The voiced labio-velar approximant is a type of consonantal sound, used in certain spokenlanguages, including English. It is the sound denoted by the letter 〈w〉 in the English alphabet; likewise, the symbol in the International Phonetic Alphabet that represents this sound is 〈w〉, and the equivalent X-SAMPA symbol is w. In most languages it is a labialized velar approximant[ɰʷ], and the semivocalic counterpart of the close back rounded vowel[u]. As labio-velar consonants do not easily fit into consonant charts with only labial and velar columns, [w] may be put in either the velar column, (bi)labial column, or both, though the latter is rare outside of the official IPA chart; the placement may have more to do with phonological criteria than phonetic ones.
Martínez-Celdrán, Eugenio; Fernández-Planas, Ana Ma.; Carrera-Sabaté, Josefina (2003), "Castilian Spanish", Journal of the International Phonetic Association33 (2): 255–259, doi:10.1017/S0025100303001373