"A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, or mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head." The most common uses of screws are to hold objects together and to position objects. There are many screws for a variety of materials; materials commonly fastened by screws include wood, sheet metal, and plastic. The differentiation between a screw and a bolt is that the latter is designed to accept a nut.
A screw is a combination of simple machines: it is, in essence, an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts as a wedge as it pushes into the fastened material, and the shaft and helix also form a wedge at the point. Screws may be self-threading (or self-tapping), designed to cut a helical groove in a softer material as the screw is inserted. The internal thread helps pull the fastened materials together and prevents pull-out.

A screw will usually have a head on one end that allows it to be turned with a tool. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions. A carriage bolt has a domed head that is not designed to be driven. A set screw may have a head the same size as or smaller than the outer diameter of its thread; a set screw without a head is sometimes called a grub screw. A J-bolt has a J-shaped head that is sunk into concrete to serve as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is called the shank; it may be fully or partially threaded. The distance between each thread is called the pitch.Most screws and bolts are tightened by clockwise rotation, which is called a right-hand thread. Screws with a left-hand thread are used in exceptional cases, such as where the screw will be subject to counterclockwise torque, which would tend to loosen a right-hand screw. For this reason, the left-side pedal of a bicycle has a left-hand thread.

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