S-trap

An S-trap is a P-trap's older, dangerous cousin—instead of the bend rising after the lowest point, the pipe drops again, creating an S profile when viewed from the side. Water sits in both bends, and when drainage occurs upstream, the lower seal siphons dry, killing the trap and letting sewer gas into the building.

You'll find S-traps in old houses, especially under sinks and floor drains. They worked by accident more than design, and any plumber who's smelled the results knows why they're gone. Modern code doesn't allow them on any fixture.

If you hit an S-trap in renovation work, replace it with a P-trap and vent it properly. Don't argue—it's a code violation and a health hazard.