A castle nut has slots or notches cut into one end. Castellated nuts are formed with a round section at the top where the slots are located. The flat sides of castle nuts extend fully from the top to the bottom of the nut. Castle nuts are designed to use a pin that fits through the slots and through a hole in the screw to which the nut is attached. Castle nuts allow the cotter pin to be confined close to the margins of the nut itself, providing added secureness. This pin prevents the nut from turning and loosening. Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.
