The spiral staircase has an inclined plane, sometimes known as a ramp. The inclined plane is formed by the stairs that wrap around the exterior wall of the chimney. They symbolize a screw machine, together with the chimney. As the staircase wraps around the outside of the chimney, it rises higher than the previous step. This makes the staircase easier to climb for people using wheelchairs or who are unable to climb many steps at once. In addition to being easy to use, the design of the spiral staircase also serves to stabilize the house against earthquakes. Since the staircase is wrapped around the inside of the chimney, if the chimney is pulled down by an earthquake, the staircase will be forced up toward the roofline, where it can be used as handholds for people to escape through windows or doors. This is how these types of staircases are also called "chimney escapes". Furthermore, since the staircase is curved, any weight on it will not cause it to collapse. This is not the case with a straight staircase which would be very difficult to climb in situations where there is no door at the top or bottom of the stairs. If someone fell down a straight staircase, they would likely break their neck when trying to save themselves from being hit by another falling person.