A P-trap is the U-shaped bend of drain pipe under a sink, tub, or shower that holds a small plug of water after every use. That water seal blocks sewer gas from entering the home, while the trap arm connects the fixture to the vented drain system. Plumbing codes require a trap on every fixture drain.
Why it matters to you
Many sink clogs and mystery sewer smells start at the trap: hair and grease collect in the bend, and traps under rarely used drains evaporate dry and let sewer gas into the room.
Practical guidance
Run water in seldom-used sinks, tubs, and floor drains about once a month to refill the seal. Place a bucket underneath before loosening a trap, and do not open one after pouring in chemical drain cleaner, since it will be full of caustic water.