Plumbing systems are composed of pipes and fittings. Metal or plastic pipes are joined by a variety of fittings designed to couple lengths in a straight line, turn corners, branch in two directions, reduce or enlarge pipe size, or connect to some type of fixture.

Pipes are made from several different metals and plastics. You can often identify a pipe's purpose by its size and makeup. Indoor water supplies generally are copper or galvanized iron pipe in diameters of 1/2 inch, 3/4 inch, or 1 inch; some water supplies in these diameters are made of plastic pipe. Gas piping is also this size but is usually made of black (uncoated) or galvanized iron pipe.

Smaller-diameter, flexible copper or plastic tubing is used for water supplies that feed ice makers, hot water dispensers, water filters, and the like. Fittings may be brass or plastic. You'll also find flexible (sometimes ribbed) pipes serving from a small wall valve to toilets and faucets and flexible piping rated for delivering gas from valves to water heaters, dryers, and other gas appliances.

Larger-diameter pipes, from 1 1/2 inches to 4 inches, are used in the drain-waste-vent (DWV) system. A 4-inch or larger plastic or cast-iron pipe usually serves the main soil stack, the waste and vent line that serves toilets and other bathroom fixtures. Pipes that are 1 1/2 inches and larger in diameter generally serve other waste and vent lines; light-gauge plastic pipe from 1 1/4-inch to 1 1/2-inch in diameter is sometimes used for built-in vacuum-cleaning systems.