A double p trap is a plumbing fixture consisting of two p-shaped sections connected by a short straight section. The first p-trap is installed under the sink, while the second p-trap is installed under the drainpipe. The purpose of the double p trap is to prevent sewer gases and odors from entering the home. The first p-trap creates a water seal that blocks the gases from entering the home, while the second p-trap provides a secondary barrier. Double p traps are typically made of PVC or ABS plastic and are available in a variety of sizes.