The system of pipes that carries water and waste to a sewer line or septic tank is called the drain-waste-vent (DWV) system.
As the name implies, it has three components: Drain lines collect water from sinks, showers, and tubs; waste lines carry waste from toilets; and vent lines exhaust sewer gases and provide the necessary air pressure to allow wastes to flow freely. All drain and waste lines slope slightly downward from the fixture toward the sewer or septic system. Water and wastes are carried by gravity.
The pipes are large in diameter—typically 1 1/4 inches to 4 inches—to minimize the possibility of blockages. The main soil stack for toilets is normally a 4-inch pipe; showers usually have 2-inch pipes.
Sinks, lavatories, bathtubs, and laundry tubs may be served by 1 1/4 to 2-inch pipes. Though some old homes may have pipes made of lead, most drain piping is ABS plastic, cast iron, or copper. Some vent pipes are galvanized iron.
To operate properly and safely, each drain must be served by a vent line that carries sewer gases out through the roof. Several vents may be connected together and joined to one larger soil stack as long as there is no drain above the connection point. Or vents may pass through the roof on their own. Wherever vent pipes penetrate the roof, special flashing protects against roof leaks; for a closer view of vent flashing, see Roof Flashing. 
All waste lines should have cleanouts at easily accessible locations. A cleanout is simply a Y-shaped fitting in the line that is capped off. If a blockage occurs in the drain pipe, a cleanout offers an easy place for a plumber to snake out the line.
To prevent sewer gases and odors from entering the house, drains are protected by traps. A trap is a curved section of drain pipe that fills up with water, providing a seal.
Drains that penetrate a wall have a P trap, and those that go through the floor have an S trap. The water held by the trap is replaced each time the fixture is used. For more about traps, see Sink Traps.
