Trusses were probably first used in primitive lake dwellings during the early Bronze Age, about 2500 BC. The first trusses were built of timber. The Greeks used trusses extensively in roofing, and trusses were used for various construction purposes in the European Middle Ages. Andrea Palladio’s I quattro libri dell’architettura (1570;Four Books on Architecture) contained plans for timber trusses. A major impetus to truss design came in the development of covered bridges in the United States in the early 19th century. Cast iron and wrought iron were succeeded by steel for railroad truss bridges. The two systems most commonly used are the Pratt and the Warren;in the former, the sloping web members are parallel to each other, while, in the latter, they alternate in direction of slope. Steel trusses are also used in many kinds of machinery, such as cranes and lifts, and in aircraft wings and fuselages.