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.
Some screw threads are designed to mate with a complementary thread called an internal thread (internal thread), often in the form of a nut object with an internal thread. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common use of screws is to hold things together and position items.
A screw usually has a head on one end that allows it to be turned with a tool. Common tools for driving screws include a screwdriver and wrench. The head is usually larger than the body of the screw, which prevents the screw from being driven deeper than the length of the screw and provides a bearing surface.
There are exceptions. A carriage bolt has a domed head that is not intended to be driven in. A set screw may have a head equal to or smaller than the outside diameter of the screw threads; A headless set screw is sometimes referred to as a grub screw. A J-bolt has a J-shaped head that is sunk into concrete to serve as an anchor bolt.
The cylindrical part of the screw from the bottom 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.