S-trap

An S-trap is a plumbing fitting shaped like the letter S, used beneath sinks and other fixtures to prevent sewer gases from entering a building.

Water collects in the curved lower portion, creating a seal that blocks odors while still allowing wastewater to drain.

S-traps have largely been replaced by P-traps in modern plumbing codes because the S-trap design can lose its water seal through siphoning, potentially allowing gases to escape.