An arch bridge is a sort of architectural construction that is supported by a curved, semi-circular shape. Most arches distribute weight evenly and are a popular choice for bridges because they may span long distances without the requirement for poles or other weight-bearing systems to be buried into the ground. As well as being easy to build, an arch bridge provides good visibility because there are no side walls to obscure the view. Arches come in two main varieties: continuous and segmental. A continuous arch remains single curve throughout its length, while a segmental arch has separate curves at each end. Continuous arches are usually made from steel or concrete, while segmental arches can be made from anything that will hold its shape, such as wood or stone. The type of arch used affects how it is constructed. For example, a continuous ribbed arch consists of alternating layers of heavy material and light material stacked on top of one another like bricks. The thickness of these layers determines the height and strength of the bridge. While this type of arch is strong enough to support traffic, it is also heavy and expensive to build. A cheaper alternative is the segmental arch. This has a flat base plate with four legs attached. The ends of each leg are shaped like half cylinders with flat tops. These segments are tied together with metal rods so that they form a complete circle when placed side by side.