Castellated nuts also called castle nuts comes in four variety:
・Type 2 high with a groove part protruding upward from the hexagonal part, theheight of the nut is the same as a standard nut of the same size. Type 2 high is the most common type with a wide range of sizes. 
・Type 1 high, the groove part has a notch in the hexagonal part, the nut is a type with a height, mainly for small diameter sizes 
・Type 2 low, the groove part protrudes from the hexagonal part and the height of the nut is low. Used where the height of the nut is a concern.　 
・Type 1 low, the groove part has a notch in the hexagonal part and the height of the nut is low. Used where the height of the nut is a concern.