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The voiced bilabial fricative is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents this sound is ⟨β⟩, and the equivalent X-SAMPA symbol is B. The official symbol ⟨β⟩ is the Greek letter beta.
This letter is also often used to represent the bilabial approximant, though that is more precisely written with a lowering diacritic, that is ⟨β̞⟩. That sound may also be transcribed as an advanced labiodental approximant ⟨ʋ̟⟩, in which case the diacritic is again frequently omitted, since no contrast is likely. It has been proposed that either a turned ⟨β⟩ or reversed ⟨β⟩ be used as a dedicated symbol for the bilabial approximant, but despite occasional usage this has not gained general acceptance.
It is extremely rare for a language to make a phonemic contrast between the voiced bilabial fricative and the bilabial approximant. The Mapos Buang language of New Guinea contains this contrast. Its bilabial approximant is analyzed as filling a phonological gap in the labiovelar series of the consonant system rather than the bilabial series. Proto-Germanic and Proto-Italic are also reconstructed as having had this contrast, albeit with being an allophone for another consonant in both cases.
The bilabial fricative is diachronically unstable and is likely to shift to.