O-rings are used throughout a pools plumbing to create watertight seals. You find large O-rings sealing the joints of filter to small O-rings sealing rotating valve stems. O-rings are used to create water tight seals around threaded fittings.
An O-ring is a doughnut shaped ring molded from an elastomer.
For more details about O-Rings see the Parker O-Ring Handbook
O-Rings? O-Yeah! How to Select, Design, and Install O-Ring Seals is a great video about O-Rings.
