A truss is a structural framework composed of several members, so arranged and joined together that they form a series of triangles, thus transferring the load imposed at any point on the framework along the members to the walls, columns, or other vertical supports at the ends of the truss. No intermediate partitions, columns, or other centre vertical supports are necessary in a standard roof-truss arrangement. Trusses are extensively used as the framework of roofs, as floor joists, over large wall openings, and in other places where structural support is necessary to carry the load over a long span. Trusses are preassembled and set in place as prefabricated units.