Editing Plumbing fixture (section)

====Traps and vents====
[[Image:Drain cover with trap.jpg|thumb|350px|This drain cover has a container underneath (which can be taken out for cleaning and revealing another container below) acting as a trap. Water inside the container forms a seal when the cover is in place. Positive air pressure will push the cover up, acting as an early warning device. The underside of the cover (centre image) is kept moist by condensation occurring and insects that go back up the drain pipe get stuck to the walls of the cover.]]

All plumbing fixtures have [[Trap (plumbing)|traps]] in their drains; these traps are either internal or external to the fixtures.  Traps are pipes which curve down then back up; they "trap" a small amount of water to create a water seal between the ambient air space and the inside of the [[Plumbing drainage venting|drain system]].  This prevents [[sewer gas]] from entering buildings.  

Most water closets, bidets, and many urinals have the trap integral with the fixture itself.  The visible water surface in a toilet is the top of the trap's water seal.

Each fixture drain, with exceptions, must be vented so that negative air pressure in the drain cannot siphon the trap dry, to prevent positive air pressure in the [[sanitary sewer|sewer]] from forcing gases past the water seal, and to prevent [[methane|explosive sewer gas]] buildup.