Bolt is a type of fastener formed of a metal rod that has an enlarged head at one end and a screw thread at the other. Bolts may be screwed directly into a threaded hole in the part they are to hold, or they may be held in place by nuts. A nut is a block of metal with a hole in the center. The hole is threaded to match the threads on the bolt.
Machine bolts have a square or hexagonal (six-sided) head. They are made in sizes from 1/4 inch (6 millimeters) in diameter to 6 inches (15 centimeters) or more, and in a wide range of lengths. Carriage bolts, typically used to fasten wood parts, have a rounded head. A small square section beneath the head digs into the wood to prevent the bolt from turning when the nut is screwed on. Stove bolts usually have half-round or flat heads that are slotted so they can be used with a screwdriver. Eye bolts have a head that forms a loop. Expansion bolts have slotted or V-shaped pieces of metal that expand when the bolt is inserted. Lag bolts have a pointed end and wide, coarse threads.
