A nut is an accessory fastening component used with bolts, studs and some screws. In general, it has an inner hole that is threaded to receive the male threads of the fastener, and an outer hard shell, usually hexagonal or square, that can be turned with a tool called a wrench. The wrench can be turned either in a direction to tighten or loosen the bond between nut and screw/bolt.
Some nuts are intended to be turned by finger pressure. These include wing nuts, which have flanges to provide a grip, or cap nuts, which have knurled or otherwise machined cylindrical heads, which give a neat appearance.
Nuts may have additional features to make them more secure. A elastic stop nut has a flexible washer (fastener) built inside them. A castellated nut has a slot to allow a safety wire or pin to be run through it, blocking motion of the screw or bolt.
They are made from a wide range of materials. The metal of a nut should, except in carefully engineered situations, match the metal of the screw or bolt, lest they corrode due to electrolytic effects. Nuts exposed to water need to be rustless, as in stainless steel, galvanized steel, or brass. For some applications, nuts may be made of nonmetallic materials.