A tee is used wherever a branch line needs to join a main drainage run at a right angle — the standard way to add a fixture connection onto an existing line. The 4-inch size joins a same-size branch into a 4-inch main. PVC-U equal tee, sized for 4-inch (BS EN 1329 nominal equivalent), all-socket joints. Wall thickness on this bore follows BS EN 1329 soil-pipe wall thickness for this bore, and the socket geometry follows BS EN 1329-1 so it seats correctly against other soil-and-waste PVC-U fittings on the same line.