Evolution of [v] to [b] and vice versa

Question

There are many examples that show that two phones [v] and [b] are related:

	b	v	Meaning
Old English to New English	* habban	have	have
Middle Persian to New Persian	varan	baran	rain
Middle Persian to New Persian	vehtar	behtar	better

*Related to German haben

Besides, there is another relation between these phones.In the Cyrillic alphabet, the sign B is pronounced [V].

The phonetic characteristics of these two phones is as follows:

	Place of articulation	Manner of articulation
v	Labiodental	Fricative, Voiced
b	Bilabial	Stop, Voiced

Given that they don't have much in common, I'd like to know:

• How is the connection between these phones justified?

Answer 1

The differences between [b] and [v] are fairly trivial between from a historical and phonetic perspective. The count of shared categories in the IPA chart isn't a good way of judging similarity (and the IPA doesn't claim to embody all relevant concepts of "category"). They are voiced oral labials: you can add "-dental" or "bi-" to be more precise. They are so similar that it is often hard to distinguish them.

Historical change operates by a series of changes, not leaps from Proto-X to Modern Y, and generally the progression is from [b] to something like [β] and [ʋ]. In some dialects of Tigrinya, /b/ becomes "[β]" in post-vocalic position, and this is optional but usually happens, but where it is extremely difficult to sharply categorize tokens as having [b] versus [β] (there is no contrast, you just have to look closely at the waveform). The difference has to do with the timing of the closure going into the consonant plus the total duration of the consonant, and very often it's a coin toss whether [b] or [β] is a better transcription.