When thinking of a plumbing system for your home or dwelling, you first must ensure that waste efficiently and effectively exits the structure into the municipal sewer system or septic tank. Your plumbing system, however, is a two-way street. A specialized fitting called a p-trap creates a powerful barrier that stops sewer gas and odor from entering your home. The trap is employed in any plumbing system that uses a drain and accepts sanitary waste from tubs, showers and sinks.