A manual door closer stores the energy used in the opening of the door in a compression or torsion spring and releases it to close the door. Some closers allow for adjustment of the strength of the spring, making it easier or more difficult to push the door open. To limit the speed at which the door closes, most door closers use hydraulic (oil-filled) dampers, although spring mechanisms may also be used for damping. Some closers incorporate a back-check facility which prevents the door from being opened too fast; this is useful for, for example, external doors where there is a danger of wind catching and blowing it open, something which could otherwise cause damage to the doorset and/or objects and/or people near it. The speed at which the door closer closes the door may be adjustable by up to three adjustment valve.