A Butterfly Valve is an inline valve with a metal disc mounted on a rod which rotates open or closed with a quarter turn. When the valve is closed, the disc is turned perpendicular to the line so that it completely blocks off the flow of liquid. When the valve is fully open, the disc is rotated a quarter turn so that it is parallel to the line and allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow. Unlike other valves, the disc is always present within the flow, so it induces a pressure drop, even when open. Butterfly valves are generally favored because they cost less than other valve designs, and are lighter weight requiring less support.