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.
The bilabial fricative is diachronically unstable[clarify] and is likely to shift to [v].
The sound is not used in English dialects except for Chicano English, but it can be produced by approximating the normal English [v] between the lips.
^Phonetic studies such as Quilis (1981) have found that Spanish voiced stops may surface as spirants with various degrees of constriction. These allophones are
not limited to regular fricative articulations, but range from articulations that involve a near complete oral closure to articulations
involving a degree of aperture quite close to vocalization
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