Reynolds 531

Reynolds 531 (pronounced 'five-three-one') is a brand name, registered to Reynolds Technology of Birmingham in the United Kingdom, for a manganese–molybdenum, medium-carbon steel tubing that was used in many quality applications, including race car chassis, aircraft components and, most famously, bicycle frame tubing. It is one of a number of tubing types developed by Reynolds.

Reynolds 531 "was the standard of excellence for many decades" among bicycle frame-building materials. And because of the availability of a wide range of butting, diameters and thicknesses of tubes, along with different stays and fork blades, it became the tubing of choice for most frame builders.