A structure forming a road over a river, etc. or allowing passage between two points above the ground. A bridge can be a simple plank or single arch or an elaborate architectural structure supported by arches, chains, girders, piers, etc. They are made of many different materials. The first bridges were natural, such as arches of rock. The first manmade bridges were flat stones or tree trunks laid across a stream to make a girder bridge. Three types of bridge - beam or girder, arch, and suspension - have been known and built from the earliest times.