AndrasBP said: Interesting. Do you know if they substituted [f] with a [x] in other words, too? Click to expand...

They basically substituted [f] with [xʋ]~[xw], but before other consonants and in the coda it was just [x]. It was a predominant model to subsitiute [f] through the Russian dialects which didn't have it. Using [p] for that purpose also existed but was generally marginal (however it was the only variant in Ukrainian and Belarusian until they actually loaned [f]).For those Russian dialects which had [v] producing [f] was not a problem (it was regularly produced in positional devoicing of [v] anyway), but the trouble is that a lot of Russian dialects didn't have [v] either (in the Old Russian dialects of the Vladimir area it was a development of earlier [ʋ] and it was slowly spreading through other dialects since then, but many other dialects retained the original [ʋ] or, more frequently, produced [u̯]~[w] from it, which was predominant in South Russian dialects until recently).