All- My understanding is that the term "butt" refers to a change of thickness. So the classic seat tube has two different wall thicknesses and is "single butted". A "double butted" tube could have three different wall thicknesses, or two as classic 531 did. The two transitions is what made it double butted", not what the resulting three sections' walls are. A "Triple butted" tube would have a third transition to make four distinct sections, of what ever wall the manufacture wanted.
As long as we're talking about tubing and wall thickness, I was taught that the blades and stays of classic 531 started as a straight tube with it's wall tapering from a thick wall to a thin wall. So that when the thin wall end's diameter was reduced, the wall increased in thickness and the finished tapered tube's wall was pretty consistent in thickness. Now Columbus started with straight tubes of straight gauge. When they were tapered down the small diameter end's wall thickness grew. The finished tube was now single butted. It was cool to attribute the claimed greater stiffness of a race built Columbus frame to this wall thickness difference (when comparing to the common 531 English built bikes). I always thought it was more due to the handling differences between the geometry that typically existed then.
> Message: 2
\r?\n> Date: Sun, 23 Aug 2009 19:56:28 -0700
\r?\n> From: donald gillies <gillies@ece.ubc.ca>
\r?\n> Subject: Re: [CR] Did any company ever make quadruple-butted tubing?
\r?\n> To: <classicrendezvous@bikelist.org>
\r?\n> Message-ID: <20090824025628.3F92C10798@lvs1-r3.ece.ubc.ca>
\r?\n> Content-Type: text/plain
\r?\n>
\r?\n> The interesting thing about butted is "double butted" means it tapers
\r?\n> at one end, then tapers back at the other end of the tube. So double
\r?\n> realy means the taper changes only twice. I think "triple butted"
\r?\n> means it tapers inwards at 2 places, then tapers outwards once.
\r?\n>
\r?\n> The main purpose of butting is that (a) it's designed to allow thin
\r?\n> and springy tubes but also (b) it's designed to be thick at the ends
\r?\n> for the joinery of the tubes. So, the thick ends of the tubes should
\r?\n> have a long enough butt (typically 75-100mm or 3-4") so that the lug
\r?\n> points do not dig into the ultra-thin part of the tubing. So an
\r?\n> italian long-point lugged frame should probably have longer butts than
\r?\n> a bocoma pro (short-point) lug set.
\r?\n>
\r?\n> The benefits of quad-butting are imho doubtful. On some bikes like
\r?\n> Miyata, I believe they put the downtube shifter bosses after the first
\r?\n> (but not second) butt. Tearing at the shifter bosses is a very common
\r?\n> downtube failure mode. So I can perhaps understand triple butting on
\r?\n> a downtube, but I seriously doubt if it's needed when you could use
\r?\n> low-temperature silver-solder for installing downtube shifter bosses,
\r?\n> bottle bosses, and pump pegs, etc., to minimize heating and weakening
\r?\n> of the tubes at the thinnest points.
\r?\n>
\r?\n> - Don Gillies
\r?\n> San Diego, CA, USA