An R-clip, also known as an R-pin, R-key, hairpin cotter pin, hairpin cotter, bridge pin, hitch pin clip or spring cotter pin, is a fastener made of a durable but flexible material, commonly hardened metal wire, resembling the shape of the letter "R". There is also a double loop variety when a wider range of suitable shafts is required, and to facilitate the attachment of a retaining tether to prevent the pin's loss when removed. It also spreads out the stresses more evenly for a longer life. DIN 11024 is example of standard that defines a set of R-clips.