In modern plumbing, a drain-waste- vent (or DWV) is a system that removes sewage and grey water from a building and vents the gases produced by solid waste. Waste is produced at fixtures such as toilets, sinks and showers, and exits the fixtures through a trap, a dipped section of pipe that always contains water. All fixtures must contain traps to prevent gases from backing up into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a soil stack, or soil vent pipe, which extends from the building drain at its lowest point up to and out of the roof. Waste is removed from the building through the building drain and taken to a sewage line, which leads to a septic system or a public sewer. Cesspits are generally prohibited in developed areas.