A screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Screws and bolts are made in a wide range of materials, with steel being perhaps the most common, in many varieties.