Pneumatic cylinders (sometimes known as air cylinders) are mechanical devices which produce force, often in combination with movement, and are powered by compressed gas (typically air). To perform their function, pneumatic cylinders impart a force by converting the potential energy of compressed gas into kinetic energy. This is achieved by the compressed gas being able to expand, without external energy input, which itself occurs due to the pressure gradient established by the compressed gas being at a greater pressure than the atmospheric pressure. This air expansion forces a piston to move in the desired direction. Single acting pneumatic cylinders (sac) use the force imparted by air to move in one direction (usually out), and a spring to return to the "home" position. Double acting pneumatic cylinders (dac) use the force of air to move in both extends and retract strokes. They have two ports to allow air in, one for outstroke and one for in stroke. Although sacs and dacs are the most common types of pneumatic cylinder, the following types are not particularly rare: rotary air pneumatic cylinders: actuators that use air to impart a rotary motion. Rod less pneumatic cylinders: actuators that use a mechanical or magnetic coupling to impart force, typically to a table or other body that moves along the length of the cylinder body, but does not extend beyond it.