There are six main types of bridges: beam bridges cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges. Beam bridges are horizontal beams supported at each end by piers. Cantilever bridges are built using cantilevers - horizontal beams that are supported on only one end. Most cantilever bridges use two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the centre. Arch bridges are arch-shaped and have abutments at each end. The weight of the bridge is trusted into the abutments at either side. Suspension bridges are suspended from cables, which hang from towers that are attached to caissons or cofferdams. Cable stayed bridges are also held by cables. However less cable is required and the towers holding the cables are proportionately shorter. Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed girders for the sides. Bridges may be classified by how the four forces of tension, compression, bending and shear are distributed through their structure. Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter.