plumbing, System of pipes and fixtures installed in a building for the distribution of potable water and the removal of waterborne wastes. Plumbing is usually distinguished from water and sewage systems, which serve a group of buildings or a city. Improvement in plumbing systems was very slow, with virtually no progress made from the time of the Roman system of aqueducts and lead pipes until the 19th century. Eventually the development of separate, underground water and sewage systems eliminated open sewage ditches. Present-day water pipes are usually made of steel, copper, brass, plastic, or other nontoxic material. A building’s waste-disposal system has two parts: the drainage system and the venting system. The drainage portion comprises pipes leading from various fixture drains to the central main, which is connected to the sewage system. The venting system consists of pipes leading from an air inlet (usually on the roof) to various points within the drainage system; by providing the circulation of air within the system, it protects the trap seals of fixtures from siphonage and back pressure. See also sewage system, water pollution, water-supply system.