Tees are used for horizontal or vertical ducting. A tee is a straight section of duct with a mortise in it. Notes For D 100-315 A=40 mm, for D 355-800 A=60 mm, for D 900 and above A=100 mm. For D 100-355 t=0,55 mm, for D 400-900 t=0,7 mm, for D 1000 and above t=1,0 mm.