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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.
10-32 is a fine thread, but it is listed under coarse thread because it is very common.
A nut with a finished top that covers the end of the bolt. Bolts must be in the proper length.
Questions? Comments? Suggestions for improved service?