A main supply pipe connected to the water company's main or to a well on your property supplies water to your house. If you've ever seen water gushing from a fire hydrant, you have some idea of the pressure moving the water along.

Once the water arrives at your house, the main supply divides into one system of pipes for cold water and another for hot water. Parallel hot and cold water pipes slope slightly to the lowest points so the pipes can be drained through a valve or faucet.

Waste drainage systems take advantage of gravity to channel waste to the sewer line. The soil stack, a vertical run of pipe 3 to 4 inches in diameter, carries waste to a main drain, usually underneath the house, which empties to a sewer or septic tank.

Vents prevent sewer gasses from seeping into your house, while traps'water-filled bends in pipes'keep gasses from escaping up the drain. Vents branch off below the soil stacks while gasses vent through the roof. Plumbing generally is vented with a main vent stack, or there may be more than one if fixtures are widely separated.

A pipe's size and material can serve as a fairly good indicator of its function. White plastic, copper, and galvanized (silver-toned) pipes that are from 1/2 inch to 1 inch in diameter generally carry water, though some galvanized steel, black steel, and flexible copper pipes of the same sizes may carry gas.

Large-diameter (1 1/2 inch and larger) black plastic, cast-iron, and copper pipes are often used for the drain-waste-vent system.