This graphic shows the basic paths of a typical home's sewer lines -- key parts of which carry no water at all, but only air to provide proper venting.
(1) All drain pipes should be connected to a network of ventilation pipes that go up through the roof. Venting prevents sewer gases from drifting out of drains into living quarters. When having a home inspected before making a purchase offer, make sure the inspector confirms that all venting pipes are present.
(2) Most plumbing fixtures have curved "trap" sections that hold a little water forming an airtight seal to keep gases in. This section of pipe is easily removed to clear clogs.
(3) Toilets also have a trap, which is what keeps standing water in the bowl. The toilet drain is the largest drain pipe in the house.
(4) All household drains meet below the house in a main drain that carries the wastewater to the municipal sewer lines or to a septic system. A Y-shaped pipe in a basement or crawlspace provides clean-out access.