What Is a P-Trap?

The U-shaped pipe under a sink that holds a small plug of water to block sewer gas from rising into the room. It unscrews by hand at two slip nuts, which makes it the easy access point for clogs that sit below the drain opening.

The water sitting in the bend is the entire point: it's a liquid seal. If a sink is rarely used, that water evaporates and sewer smell drifts up; the fix is simply running the tap for thirty seconds. A P-trap that drips at the joints needs its slip nuts snugged (hand-tight plus a quarter turn) or its beveled washers reseated. The washers cost pennies and are the usual culprit.

Opening one for a clog: put a bucket under it, unscrew both slip nuts by hand, and the trap comes free full of water. Dump it, scrub the inside, check the wall-side pipe within reach, and reassemble. Plastic (PVC) traps are what most US homes have and are fully hand-serviceable; chrome-plated metal traps corrode from the inside and often need replacing outright ($8–$15) rather than cleaning.