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Nut size (A.K.A. diameter) is the size bolt that the nut is used with. The same is true of the thread count. Thus a 1/4"-20 nut fits a 1/4"-20 bolt.
NOTE: While 12tpi is the fine thread (UNF) standard, 14tpi is far more common in actual use.
Cotter or split pins have two tines which are bent apart to hold them in place.
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