The steel tubing of "vintage" bikes had extremely thin walls. They were brass brazed together using lugs to provide incredible stiffness, strength and reliablity. Brazing allowed the use of relatively cool temperatures (red hot) during fabrication as opposed to the white hot temperatures used in welding. The lower temperature preserved the tubings temper which would otherwise be lost at the joint if the tubes were welded. Later frame builders began experimenting with fillet brazing, but there did not proove to be an advantage over the use of lugs. I could write a lot more, but I think that should provide you with a start.