Master link

A master link or quick-release link is a roller chain accessory that allows convenient connection and disconnection of a chain without the need for a chain tool. It acts as a set of the chain's outer plates, so joining two sets of the chain's inner plate ends. Such master links may or may not be re-usable. A chain tool is nonetheless needed to adjust a chain's length, for example to shorten a new chain before connecting its ends. They are used on bicycles and motorcycles.