Castle nuts are fastening devices with slots or notches on one end that allow for the insertion of a cotter pin or safety wire to keep the nut from loosening. They are commonly used in automotive, aerospace, and industrial applications where vibration and movement could cause traditional nuts to loosen over time. Castle nuts are made of various materials such as steel, stainless steel, brass, and aluminum. They are available in different sizes and shapes to fit different types of bolts and screws. Castle nuts are also known as slotted nuts or castellated nuts.