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.
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This item is a semi-standard size and is therefore more expensive.
USS pattern is the most common type of flat washer.
SAE pattern is a tighter fit on the bolt and a smaller outside diameter.
A high crown type of cap nut used for appearance. Bolts must be in the proper length.
A nut with a finished top that covers the end of the bolt. Bolts must be in the proper length.