In a generally spot-on message from Don Gillies, I have doubts about two statements, so I'm going to <snip> the rest to comment on them:
<snip>
Don: The main purpose of butting is that (a) it's designed to allow thin and springy tubes but also (b) it's designed to be thick at the ends for the joinery of the tubes. So, the thick ends of the tubes should have a long enough butt (typically 75-100mm or 3-4") so that the lug points do not dig into the ultra-thin part of the tubing. So an italian long-point lugged frame should probably have longer butts than a bocoma pro (short-point) lug set.
Harvey: I've never heard the assertion about lug points digging into thin tubing; it's all very soft at temperature. I think the real point, which you allude to, is that the thicker butt was supposed to increase the heat capacity and thus limit the local temperature rise when doing hand-held (or hearth) brazing.
Don: The benefits of quad-butting are imho doubtful. On some bikes like Miyata, I believe they put the downtube shifter bosses after the first (but not second) butt. Tearing at the shifter bosses is a very common downtube failure mode. So I can perhaps understand triple butting on a downtube, but I seriously doubt if it's needed when you could use low-temperature silver-solder for installing downtube shifter bosses, bottle bosses, and pump pegs, etc., to minimize heating and weakening of the tubes at the thinnest points.
Harvey: Agreed on the silver solder for tiddly bits, but I've never seen a frame with tearing at the shifter bosses. Accidents? Real monster abuse like kicking the thing? I have seen frames that failed with transverse cracks behind the head tube, as though heat had been too localized (but could have had other causes), and there was some talk that Nishiki Competition frames tended to suffer downtube failures. Below the shifter bosses, as I recall, but this is veyr fuzzy.
Harvey sachs
mcLean va