A trap is a vessel which contains water, its purpose is to prevent the passage of sewer gas and other foul odors from the sewer into the house, or to prevent the entrance through the house fixtures of gas and noxious odors that may be formed between the main trap and the house fixtures. The water seal of a trap should not be less than 1 to 2 inches.

The seal of a trap may be broken in different ways, viz: by syphonage, evaporation, back pressurage and momentum or the action of the waste itself as it may pass off with considerable force.

A good trap should have a good seal, it should be non-syphonable, self-cleaning and have as few corners or places where dirt or refuse may collect as possible.

The S-trap and the drum or cylinder trap are two forms most used.

The back pressure or gas from the sewer will saturate the water in a trap with sewer gas, therefore all traps should be back-vented from the sewer side of the siphon and at the highest point of the same.

Traps should always be counter-vented, principally to prevent syphonage, to ventilate the plumbing system and to relieve back pressure.

Counter-venting. A counter-vent is a pipe by means of which a trap is supplied with air, to prevent the partial or total syphonage of the trap and also ventilate the plumbing system of the house.

Counter-vents from fixture traps should always be carried into the main air-pipe and higher than the top of the fixture or else directly through the roof.

The counter-vent from a water closet should always be vented from the highest point of the syphon and never from a lower point where the flushing action of the closet would throw waste matter into the entrance of the counter-vent or at any point where the waste would be liable to settle in the vent-pipe.