Before nuts had nylon or metal inserts to make them "self-locking," a pair of low-profile nuts (about half the height of a standard nut) were installed together to create a type of locknut. The first nut was held in place with a wrench while the second nut was tightened down or "jammed" against it to lock the two nuts in place. Their primary purpose is to prevent vibration from loosening a bolted joint. Occasionally, Jam Nuts are used when a standard nut will not fit. G.L.HUYETT™ Hex Jam Nut, Measurement System: Imperial, 3/4 in Thread, Hot Dipped Galvanized, Left Hand Thread