What are the usual conventions for “ab” versus “a” before consonantal ‘i’ or ‘u’? My observation has been that “ab” occurrs before words like “Iulio” whereas “a” is placed before “uento,” unless I’m mistaken, which I hope I am. But if this was the classical convention, did the Romans pronounce their two semi-vowels differently with respect to consonantal quality?
Clearly, this is all leading back to the ‘v’ versus ‘u’ argument, though I admit I’d much rather stick with ‘u’ without confusion.
L&S disagrees with your idea that “ab” occurs before consonantal i, although it does occur before that more often than before consonantal u. There is a very nice article in L&S about the usage and history of a, ab (etc.) under the definition. The relevant part, which I trust is well researched, is thus:
[Ab is] indeed the only [form] used before all vowels and h; here and there also before some consonants, particularly l, n, r, and s; rarely before c, j, d, t; and almost never before the labials p, b, f, v, or before m
As a matter of fact, just by searching the examples on the definition page for “j”, I find ab Jove, a jumentis, and a Jove. Not the most comprehensive survey, but maybe something. Regardless, it would not prove much even if there is a distinction between which forms favor different semivowels: if only a were used before consonantal i it would be in line with its use before other consonants; if ab were used before consonantal u it would not be unreasonable since it is sometimes used before other consonants.
