Abstract: For a rectangular waveguide the wall thickness of the narrow sides b of the rectangle is smaller than the wall thickness of the broad sides a of the rectangle with the ratio of the wall thicknesses being so chosen that approximately the same resistance to deformation is obtained in both directions. For waveguides which consist of welded plates the broad sheet metal plates 10 are cut to the nominal inner dimension and the sheet metal plates which form the narrow sides and have a lower wall thickness abut laterally against the two plates belonging to the a side.