The U-shape pipe under a sink is called a P-trap.

The purpose of a P-trap is to create a water seal that prevents sewer gas from coming into the house and to collect wedding rings, gold coins that might go down the drain before they are swept away.

A properly installed P-trap has a water seal between 2-4 inches (between the two horizontal red lines) and uses smooth wall pipe, not flexible pipe.

The plumbing configuration in the photo will result in poor sink drainage and in time become plugged with hair, soap scum, dirt, and other debris.

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