A house that comprises at least one main stack, as shown in the Drain water vent plumbing plan. The stack is a large vertical pipe (often 3 or 4 inches in diameter) that runs from above the roof to the main sewer line. Horizontal drain and vent pipes run into the main stack from the house's various toilets, faucets, tubs, and appliances. As shown in the Drain water vent plumbing plan, horizontal drain pipes must be sloped so that water does not settle in them. Venting can be a complicated process, but the basic idea is simple: A vent pipe allows air to enter behind the drain water, allowing it to flow more smoothly. Drain water can gurgle if not vented, like water coming out of an upturned thin-necked bottle.