When frames are being constructed by TIG welding the single-butted seat tube, with 0.5mm wall thickness at the top would not be strong enough to accept the heat of the welding. One method to overcome this is to use a sleeve over the outside of the tube. However Reynolds eliminate the need for this with bulge-butted seat tubes. These are double-butted tubes, with different thickness at each end. A typical tube would be 0.9/0.6/1.2mm thick. At the 1.2mm end we then bulge the tube so the inside diameter is 27.2 to take a standard seat pillar. The frame builder also has the extra thickness for the TIG welding..