Also known as the Roman arch, the Semicircular Arch forms a half-circle and is a major feature of all Roman architecture. A strong structure, this arch is often found in bridges and aqueducts. It appears to be a simple arch to manufacture, as each voussoir is identical in taper to its neighbor. The only variable is the length of the stiles or sides of the arches, which determine the height of the bridge or building that uses them. The semi-circular arch is formed by two opposite arcs meeting at a point. This point is called the center of the arch. The radius of the inner arc is equal to the radius of the outer arc. They form a half-circle with the center of their intersection being the center of the arch. Because both arcs are of the same size, the distance between their centers is always the same and equals one-half the width of the arch. The faces of the voussoirs that define the arch are usually not perpendicular, but tend to be more angled. However, because they are flat surfaces, they can be made perfectly horizontal if needed. Because of its strength and reliability, the semi-circular arch is used in many structures where appearance is not important. These include gates, doors, and windows. It is also common in bridges and buildings whose function is not related to movement of people or goods.