Abstract: Apparatus for forming a flexible tubing (S) from a single-ply or multi-ply web (B) as it is continuously advanced includes two size-defining metal plates (9, 10) which define the width of the flattened tubing and are substantially parallel to each other and are movably mounted in the machine frame (1, 2, 3) and have receiving end portions about which the web is deflected to raise edge portions of the web which are to be infolded to overlap each other. The plates are adjustable by a screw (34) or the like to control the distance between their receiving end portions. The plates are provided at their delivery end portions with straps (17) or the like for controlling the distance between the plates.