Under sink plumbing

I am rebuilding the plumbing under my sink, and just had a simple question. What is the "U" shaped piece that goes from the sink drain into the wall? What is it for?

R.F.
Little Rock AR

Dear R.F.,

The "U" shaped piece is called a "p-trap". It was designed by Thomas Crapper ( YES REALLY) in 1880.

Before this there was an S trap but it wasn't practical because it required an overflow due to constant jamming.

Because of its shape, the trap retains a small amount of water after the fixture's use. This water in the trap creates a water seal that prevents sewer gases 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 over-sized objects. For all of these reasons, most traps can either be disassembled for cleaning or they provide some sort of cleanout feature.