Abstract: Forming devices in packaging machines of the type which reforms tubular packaging material of substantially circular cross-sectional configuration into individual package containers of quadrilateral cross-sectional configuration normally consist of pairwise cooperating forming flaps which are of U-shaped cross-section. In order to prevent the packaging material from being pinched between the shank surfaces of the flaps when the flaps are caused to surround the packaging material tube, the shank surfaces are designed to be undulatory, i.e. alternatingly with projections and recesses which fit in one another and guide the packaging material tube into the U-shaped recesses of the flaps without pinching the packaging material tube.