A sanitary tee directs traffic in your home’s drain-waste-vent plumbing system to make sure wastes go down to the sewer and gases go out the vent stack.

Like other types of T fittings, it connects two pipes for a main run and one at 90 degrees for a branch run. A little different than a standard tee, it’s designed so waste will flow through it smoothly—the branch run curves toward the perpendicular main run (a similar tee with a long, sweeping curve is called a combination tee).

The main run must be vertical. The branch curves downward toward a drain. In some cases, a sanitary tee is used in place of a vent tee for plumbing vent lines—then, the branch sweeps upward instead. Sanitary tees are made for all types of drain-waste-vent pipe systems, whether cast iron, threaded, copper, or ABS plastic.