A jam nut is a type of locking nut that avoids compression of the part being fastened. It is commonly used to fasten parts that must be able to spin freely, such as bearings. It looks like a thin hex nut, but half as tall. It is threaded inside of and tightened against a full-thickness nut. This puts opposing tension on a localized segment of the screw thread, preventing either loosening or tightening of the pair.