Hex nuts are six-sided, internally threaded fasteners, mated with bolts, studs, screws, and other externally threaded products. The addition of the hex nut ensures that the bolt, etc., stays in place.

Slotted hex nuts have slots cut into the top of the nut through the centers of the opposing flats.  The slots are used for the insertion of cotter pins or safety wire to secure the nut when used with a drilled fastener.