A throttle valve is a valve that controls fluid flow by altering the throttle section or length. Parallel connection of a throttle valve with a check valve results in a unidirectional throttle valve. Both throttle and unidirectional throttle valves are simple flow control valves. In a hydraulic system with a fixed-displacement pump, when combined with a relief valve, they can form three types of throttle control systems: inlet throttle control system, return flow throttle control system, and bypass throttle control system. Throttle valves do not have flow negative feedback functionality, and cannot compensate for speed instability caused by load changes. They are generally used only in situations where load changes are minimal or where speed stability is not critical.