The curved U-shaped pipe under every sink that holds a small amount of water to block sewer gas from coming up through the drain.
A curved section of drain pipe shaped like the letter P that retains a small amount of water, creating a seal that prevents sewer gases from entering the building through the drain opening. Every plumbing fixture must have a trap of some type. The water seal in a P-trap can evaporate in unused fixtures, which is why trap primers are used for floor drains.
The inspector failed the rough-in because the laundry sink drain was installed with a prohibited S-trap instead of a P-trap.
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