Hex jam nuts are thinner than a hex nut but have the same dimension across the flats and corners. Hex jam nuts have two common applications. A jam nut can be threaded onto a bolt and run tight against the standard nut which will lock the nut in place and prevent it from backing off. Jam nuts are sometimes used for leveling purposes as well.
Since hot-dip galvanizing typically adds 2.2 to 5 mils of thickness to the threaded portion of a fastener, galvanized hex jam nuts are tapped oversize to compensate for the corrosion resistant coating on the bolts.