The active suspension and adaptive suspension/semi-active suspension are types of automotive suspensions that control the vertical movement of the wheels relative to the chassis or vehicle body with an onboard system, rather than in passive suspensions where the movement is being determined entirely by the road surface, see Skyhook theory. Active suspensions can be generally divided into two main classes: pure active suspensions and adaptive/semi-active suspensions. While adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to literally raise and lower the chassis independently at each wheel.