The nut has internal threads so that it can be tightened easily on the bolt. The size of the nut is smaller than that of the bolt. The nuts experience compression forces. It is compressive stress that leads to its failures. There are different types of nuts: Hex Nut, Nylon Insert Lock Nut, Jam Nut, Nylon Insert Jam Lock Nut, Square Nut, Cap Nut, Acorn Nut, T-Nut, Cape Nut, Castle Nut, Wingnut, Flange Nut, Slotted Nut, Coupling nut, etc. most common type of nut found today is a hexagonal nut.