The bilabial flap is an uncommon non-rhotic flap. It is usually, and perhaps always, an allophone of the labiodental flap, though it is the preferred allophone in a minority of languages such as Banda and some of its neighbors.
In the literature it has often been transcribed by a w modified by the extra-short diacritic, ⟨w̆⟩. (Since flaps are similar to brief stops, it could alternatively be transcribed as ⟨b̆⟩, but this could lead to interference between the diacritic and the ascender of the base character.)
In 2005 the International Phonetic Association adopted the right hook v symbol to represent the labiodental flap. Since then, the received transcription of the bilabial flap involves employing the labiodental flap symbol modified by an advanced diacritic: ⟨ⱱ̟⟩.
Olson, Kenneth; Schrag, Brian (2000), "An overview of Mono phonology", in Wolff, H.E.; Gensler, O., Proceedings from the 2nd World Congress of African Linguistics, Leipzig 1997, Cologne: Rüdiger Köppe, pp. 393–409Missing |last2= in Editors list (help)