Chain tensioner

A device which takes up the slack in a chain. Some use an idler wheel which can be adjusted (manually or automatically), others use a flat slide which pushes against the chain to keep it from bouncing around. Most modern units are spring loaded so that the tensioner automatically takes up the slack. Some require that you need to undo a locking nut to allow the spring to push against the chain. Afterward the lock nut needs to be secured again.