A butted tube is one where the outside of the tube is a constant diameter (usually 25.4, 28.6. 31.8, 34.9, 36.4 or 38.1 mm) and the wall thickness varies.

On double-butted tubes the thickness of the tubes at both ends is thicker than in the centre, so a typical double-butted tube (0.8/0.5/0.8) would have a thickness of 0.8mm at end (1) a taper portion, then a centre section of 0.5mm, a taper portion, and a thickness of 0.8mm at end (2).

This is used for the top and down tubes. When a lugged frame is being constructed the seat tube would be single-butted. So at the bottom bracket end the tube would be 0.8mm thick, there would be a taper portion, and then the 0.5mm would continue to the top of the tube, to allow the seat pillar to be inserted.