In by Mark Goodman

A P-shaped section of drainpipe that traps water, which prevents sewer odors from escaping through the drain and into the home.

P-trap (Wikipedia)
Water seal in drain pipe under a sink. Water enters at right, fills the trap, and continues left. Inverted siphoning occurs below the line "A".
Examples of traps

In plumbing, a trap is a device which has a shape that uses a bending path to capture water to prevent sewer gases from entering buildings, while allowing waste to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases from escaping outside through drains.

In domestic applications, traps are typically U, S, Q, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as an S-bend. It was invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend could not jam, so, unlike the S-bend, it did not need an overflow. The most common of these traps in houses is referred to as a P-trap. It is the addition of a 90 degree fitting on the outlet side of a U-bend, thereby creating a P-like shape. It is also referred to as a sink trap because it is installed under most house sinks.

Because of its shape, the trap retains a small amount of water after the fixture's use. This water in the trap creates a seal that prevents sewer gas from passing from the drain pipes back into the occupied space of the building. Essentially all plumbing fixtures including sinks, bathtubs, and toilets must be equipped with either an internal or external trap.

Because it is a localized low-point in the plumbing, sink traps also tend to capture heavy objects (such as jewelry) that are inadvertently dropped into the sink. Traps also tend to collect hair, sand, and other debris and limit the ultimate size of objects that will pass on into the rest of the plumbing, thereby catching oversized objects. For all of these reasons, most traps can either be disassembled for cleaning or they provide some sort of cleanout feature.

When a large volume of water may be discharged through the trap, a standpipe may be required to prevent impact to other nearby traps.

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