Before air can be used to perform the work required in a pneumatic system, it must be given potential energy in the form of pressure. The compressor gives air (at atmospheric pressure) high pressure by changing its volume. Compressors can be classified into two separate types based on their basic principle of operation: positive displacement and dynamic. Positive-displacement compressors confine the air within a closed space and compress it by decreasing the volume of the space. Dynamic compressors accelerate the air with rapidly turning rotor blades. Reciprocating compressors are manufactured in many varieties and are usually driven by electric motors. Compressors classified according to their operation include single- or double-acting, and single- or multistage. Reciprocating compressors are also identified by the way their cylinders are arranged. These include: vertical, V or Y, W, horizontal, angle or L, and semiradial.