phoneme vrs. allophone

[David L. Frye]

In Spanish, b and v represent a single phoneme (since they are pronounced
exactly alike in all positions) but each has (at least?) two allophones:
e.g. in the words vivir or beber, the first b/v is almost identical to
English b, whereas the second b/v is somewhere between English b and w --
the lips are slightly separated. But there is no Spanish word in which an
"initial b" falls between vowels and therefore contrasts with an
"intervocalic b", hence they are allophones not separate phonemes.