An ideal constant-force spring is a spring for which the force it exerts over its range of motion is a constant, that is, it does not obey Hooke's law. In reality, "constant-force springs" do not provide a truly constant force and are constructed from materials that do obey Hooke's law. Generally, constant-force springs are constructed as a rolled ribbon of spring steel such that the spring is in a rolled-up form when relaxed.