Proper use and maintenance of hydraulic systems is dependent on a thorough knowledge of the fluid and of the functions of the mechanical components. Regardless of the complexity or simplicity of a hydraulic system, each system contains seven basic components: a reservoir to hold the fluid supply, connecting lines to transmit the fluid power, a pump to convert input power into fluid power, a pressure control valve to regulate pressure, a directional control valve to control the direction of fluid flow, a flow control device to regulate speed or fluid flow, and an actuator to convert hydraulic power into mechanical motion. Although hydraulics is one of the oldest branches of science, it was only in recent years that knowledge of the physical laws of fluids was put to practical use in industry. The fluid most commonly used in hydraulic systems is petroleum-based, although water has found application in a limited number of hydraulic installations.