Standing water in the bowl acts as a barrier to sewer gas coming out of the sewer through the drain, and also as a receptacle for waste. Sewer gas is vented through a separate vent pipe attached to the sewer line. The water in the toilet bowl is connected to the drain by a drain pipe shaped like an extended "S" which curves up behind the bowl and down to the drain. The portion of the channel behind the bowl is arranged as a siphon tube, whose length is greater than the depth of the water in the bowl. The top of the curving tube limits the height of the water in the bowl before it flows down the drain. The waterways in these toilets are designed with slightly smaller diameters than a non-siphonic toilet, so that the waterway will naturally fill up with water each time it is flushed, creating the siphon action.