In
plumbing, a
trap is a U-, S-, or J-shaped
pipe located below or within a
plumbing fixture. An S-shaped trap is also known as the
S-bend invented by
Alexander Cummings in 1775 but became known as the
U-bend following the introduction of the U-shaped trap by
Thomas Crapper in 1880. The new U-bend could not jam, so, unlike the S-bend, it did not need an overflow. The bend is used to prevent
sewer gases from entering buildings. In
refinery applications, it also prevents
hydrocarbons and other dangerous gases from escaping outside through drains.