Below every single plumbing fixture that drains into the house system is a trap containing a water seal, intended to prevent the return of foul gases from the sewer into the living areas of house or garden. The trap takes the form of a small U-shaped loop of pipe so formed that it is always filled with water. As new water drains from the fixture it displaces the water in the trap; part of the outgoing water then remains in the loop to replace the seal and maintain the barrier against air that would otherwise drift back up from the drainage pipes.
The primary purpose of a trap is to prevent contamination from the drains, but it also provides a convenient means of access to the pipes below the fixture. If you have to unblock a pipe you have an alternative to working through the plughole of the fixture. A trap is also a safety measure against the loss of small valuable objects that get accidentally washed down the plughole: a ring or a contact lens can often be found lying safely at the bottom of the U-bend, and can simply be tipped out once you have removed the trap. Traps in which the horizontal pipe leading from the U-bend gives a resemblance to the letter P are called P-traps. You will also often find S-traps, with a vertical pipe after the bend, and self-contained bottle traps (opposite page, below) whose neat appearance makes them particularly popular for use under wash basins or anywhere else where they are openly visible. Traps may be made in any of the normal plumbing materials, such as copper, steel, chrome or plastic. It is usually desirable that the material of the trap and the following waste pipe should match, but a new plastic trap can be satisfactorily fixed to an existing pipe of any material. The same technique is used for attaching traps of both matching and dissimilar materials.