Mainly because they work at minimal force and depend upon gravity to move fluids (and entrained solids), drain-waste-vent devices use fittings whose interior surfaces are as easy as is possible. The fittings may very well be "belled" (expanded marginally in diameter) or usually formed to accommodate the insertion of pipe http://valvesandfittings46789.articlesblogger.com/10891145/what-does-valves-and-fittings-mean