Abstract: A plurality of flange portions are positioned oppositely of one another on the ends of a pair of generally rectangular sheet metal ducts. The flange portions are positioned in pairs on the adjacent ends of the ducts and each flange portion includes an upstanding wall having a pair of end portions for receiving the right angle legs of corner pieces. Four corner pieces extend into abutting relation with the upstanding walls at the four corners of the end of each duct to form a frame around the duct end portion. Each corner piece has a corner section, and a pair of legs extend angularly from the corner section. The pair of legs extend into abutting relation with the upstanding walls of adjacent flange portions on each duct end portion. In the assembled joint, the frames around the duct end portions are oppositely positioned with a gasket member positioned between the flange upstanding walls and the surfaces of abutting corner pieces. Each corner piece includes a corner and an outer edge with a corner flange.