Abstract: The invention relates to a butt joint between two circular or flat, oval air duct sections (10) each with a connection flange (32) at each end and a ring (34) clamping the connection flanges together. Said connection flanges (34) are moulded, to form one part, to the duct wall (12) of each air duct section (10) and each have an outwardly-projecting annular edge (14) and clamping legs (24) drawn together by corresponding side legs (38) of the clamping ring (34). Each annular edge (14) forms an angle of 80.degree. to 90.degree., preferably 87.degree. to 89.degree., with the associated duct wall (12). Each connection flange (32) can also have another or a plurality of supporting legs (28).