Hex nut with opposed slots cut into the top of the nut through the centers of the flats. The slots are on the end opposite the bearing surface. The slots are for the insertion of a cotter pin to secure the nut when used with a drilled shank fastener. Nuts are made from a low-carbon steel which conforms to the following chemical composition requirements: Carbon-0.47% max, Phosphorous-0.12% max, Sulfur-0.23% max. Hardness is Rockwell C32 maximum and proof load is 72,000 psi.Porteous Fastener Company