Castle nuts (castellated nut) and slotted hex nuts are differentiated by the ring around the crown. Castle lock nuts are a type of nut with slots cut into one end and an indented ring near the top (the slotted hex nut doesn’t have the indentation around the crown). Typically used in low-torque applications, such as holding a car wheel bearing in place. They typically lock by using a cotter pin (or some other mechanism) to go through the slot and a drilled hole in the thread of the accompanying shaft.

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