The plan of attachment of the various bathroom fixtures of the soil pipe must always depend on local conditions. The object is to conduct the waste water to the sewer in such a way as to give the least opportunity for stoppage and to prevent sewer gas from escaping into the house. To accomplish this purpose the pipes and traps are arranged according to a plan proposed by the architect, plumber or other person familiar with the principles of plumbing. Since these pipes are placed in the walls and under the floors, where they are not readily accessible, it is necessary that their arrangement be made with care and that the workmanship be such as to assure correct installation. In Fig. 99 is shown a common method of connecting bathroom fixtures with the sewer. The drawing shows a bathroom with the floor broken away to show the pipe connections with the bath tub, wash basin and closet. The overflow pipes _O_ and _V_ and the drain pipes _D_ and _R_ from the wash basin and bath tub empty into a large lead drum-trap _T_, set under the floor. This trap takes its name from its shape. It is set in position as dictated by the conditions under which it is used. The nickeled plate _P_, screwed into the top of the trap, comes just above the bathroom floor. This plate is easily removed in case of stoppage. It is made air-tight by a rubber ring placed under the cover and which makes a joint with the top edge of the drum. It will be noticed that the waste pipes from the bath tub and wash basin enter the trap near the bottom and discharge at the opposite side near the top. The water will stand in the trap and pipes level with the bottom of the discharge pipe and thus form a seal that prevents the escape of sewer gas. This is a common form of non-siphoning trap. It is non-siphoning because it cannot lose its seal by reason of the siphoning effect of the water as it passes through the waste pipes on its way to the sewer. Another form of non-siphoning trap is the clean sweep trap shown in Fig. 97. Such traps as Figs. 95 and 98 are siphoning traps, since it is possible, in this form of trap, for the water to be so completely siphoned that not enough remains to form a seal. The small drawing, marked _Detail L_, is another method of connecting the same arrangement of fixtures. The waste pipe enters the trap as before but discharges immediately opposite. The level of the water stands in the pipes as indicated by the dotted line. =Back-venting.=--To prevent the possibility of loss of seal by siphoning and the escape of sewer gas, traps are back-vented to the main stack or to a separate vent stack. The venting is accomplished by joining a pipe to the top of the trap or to some point in its immediate neighborhood, and connecting this with the main stack or the vent stack. The water in a trap so vented will be open to the air from both sides and consequently can never be subject to siphonic action.