A twin-tube shock or strut has two cylinders: The inner, or "Working cylinder", is where the piston and shaft move up and down. The outer cylinder serves as a reservoir for the hydraulic fluid. There are fluid valves in the piston and in the stationary base valve. The base valve controls fluid flow between both cylinders and provides some of the damping force. The valves in the piston control most of the damping. The term Gas Shock usually refers to the same twin-tube design, but with one improvement. Low pressure Nitrogen gas is added to replace any oxygen air. The effect lessens aeration and performance fade.