In building construction, a structural frame usually fabricated from pieces of metal or timber to form a series of triangles lying in a single plane. The linear members are subject only to compression or tension. The horizontal pieces forming the top and bottom of the truss are called the chords, and the sloping and vertical pieces connecting the chords are collectively called the web. Unlike a vault, the truss exerts no thrust but only downward pressure; supporting walls require no buttressing or extra thickening. Trusses have been used extensively in roofing and bridges. Wood trusses were probably first used in primitive dwellings c. 2500 BC. Wood was replaced by iron, which in turn was succeeded by steel.