P-trap

A P-trap is the curved section of pipe — shaped like the letter P on its side — installed under every sink, tub, shower, and floor drain. The bottom curve retains a small column of water at all times. This water seal prevents methane, hydrogen sulfide, and other sewer gases from entering the building through the drain opening.

Dried-out P-traps are a common cause of sewer-gas smells in homes — often in guest bathrooms or floor drains that haven't been used recently. Running water briefly refills the trap. Plumbing code requires P-traps on every fixture drain; alternatives like S-traps (no longer code-compliant) and AAVs (air admittance valves) are restricted or prohibited in many jurisdictions. P-traps also catch dropped jewelry and collect hair/debris over time, which is why they're designed to be easily disassembled (slip-nut connections rather than threaded).