Originally Posted by
JohnDThompson
Seat tubes and steer tubes are single-butted because they need to hold a post or stem at one end. Fork blades are "taper gauge" (a form of single butting) so the wall thickness remains constant after the blade is tapered. Head tubes are plain gauge. So the only truly "double-butted" tubes in a double-butted tube set are the top tube and down tube.
While this is a good rule of thumb, there were double butted seat tubes. Tange used double seat tubes on most of their double butted tubesets (#1, #2, #3, Mangaloy 2001, 900, 1000, Infinity), Ishiwata used a double butted seat tube on 017 and True Temper SP also had a double butted seat tube.