Tubing

Reynolds 531 was the de-facto steel tubing used to make hand built bike frames in the 20th Century. It was butted, meaning the tubes were thinner in the middle but still incredibly strong. Most modern bike frames are made of aluminium or carbon and whilst these are lighter they simply are not as comfortable to ride. Plus, if looked after a steel frame will last for ever (I ride one that my father bought second hand in the early 1950s!) whilst aluminium corrodes and carbon can break.