ABS double sanitary tee for tying two opposing fixture branches into a single vertical stack — the back-to-back junction used where two fixtures, like a pair of lavatories or a back-to-back bath group, drain into the same stack. Each branch inlet enters with a curved sanitary sweep that turns flow downward, so both lines drain cleanly without catching at the joint. All four ends are hub sockets sized for Schedule 40 IPS ABS, solvent-welded for a rigid, sealed stack. It meets ASTM D2661 and CSA B181.1 and carries cUPC and NSF/ANSI 14 certification, the listings an inspector checks on a residential or light-commercial rough-in across the GTA. Stocked in 1½" for paired-fixture branch work. Double sanitary tees are non-pressurized DWV only; test hydrostatically, never with compressed air or gas.