The term "jam nut" comes from its use as a secondary nut that jams the main nut. Jam nuts help to prevent loosening in cases where the bolt may be subject to vibration. In situations where you need to fasten a threaded rod or hanger bolt, a jam nut can be tightened against a regular nut in order to create a place for a wrench to be used. They can also take the place of a standard nut in cases where there is not enough exposed thread to support the thickness of a standard nut.