All plumbing fixtures have 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 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 sewer from forcing gases past the water seal, and to prevent explosive sewer gas buildup