Vitreous pipe: 5" heavy-grade vitreous Tee with door MATT90TX5For rear of stove connection. You must be able to access this part for sweeping; if your stove is in a recess you must have large left and right air gaps to facilitate access and sweeping. As with all vitreous pipe the bottom end narrows to a lesser diameter to enable insertion into other pieces of pipe (male is always on top). Key reason is that any condensation that trickles down inside pipe cannot escape.