A control member is rotatable to alter a control flow area of a primary flow path of a valve, with the control flow area being of a rectangular shape having a side defined by a wall of a valve interior, a side defined by a wall of a control member exterior, a bottom defined by a floor of the valve interior and a top defined by a roof of the valve interior. The control member exterior is so configured that rotation of the control member alters a distance between the control member exterior and the wall of the valve interior to alter the control flow area of the primary flow path, with the amount of rotation of the control member between two predetermined end positions and the alteration in the control flow area of the primary flow path being substantially linearly proportional.