An arch is a curved, self-supporting shape usually used over doorways or to support old bridges. Arches shift the downwards force at the top of the arch to a sideways force at the bottom of the arch, so there must be a strong wall holding the base of the arch up. Arches work particularly well underground where the earth holds the sideways force. When two arches are put next to each other, the sideways forces are cancelled out, so a row of arches together e.g. under a long bridge only need thin walls or pillars holding the arches up. The stone at the top center of an arch is usually a slightly different shape, or is more decorative. It is called the keystone.