A lockwasher is a split or toothed washer. It is usually installed between a nut or screw head, and a flat washer or the actual part and is used to help keep a nut or screw from loosening in service. The split washer is crushed flat and locks the nut in place by spring tension, while the toothed lockwasher, usually used for smaller bolts, provides many edges to improve the locking effect.