Check nut, Jam nut, Lock nut, a nut which is screwed up tightly against another nut on the same bolt or screw, in order to prevent accidental unscrewing of the first nut.

The idea of a jam nut is to cause the two nuts to press on the threads in opposite directions, so that the thread is clamped between them, and this clamping is not relieved if the nuts should turn slightly on the bolt, so that unscrewing is discouraged. If this condition does not exist, and it usually will not be if the thinner nut is on top and tightened second, then the top nut and after it the lower nut can loosen without restraint, and there will be no locking action. Two nuts in this case are not much better than one. The proper connection is made by tightening the jam nut snugly first, then tightening the upper nut so tightly that the stress on the jam nut is reversed as the bolt strains. The two nuts could be the same thickness, but it saves space if the lower one is thinner. The thinner nut must counter only part of the compressive force of the regular nut, which does double duty. It is surprisingly difficult to find illustrations of a jam nut used properly, or torque specifications.