Square section tubes are quite common paper automata models. They are used as push rods, slider tubes and other sorts of linkages. Making long tubes can be especially problematic - it is all too easy to glue the tube with a slight inaccuracy and end up with a twist or bend along its length. This simple technique ensures that square section or rectangle section tubes come out straight and untwisted.
Fold all the creases of the tube so that it is ready to assemble. Spread a thin layer of glue along the full length of the tab. You only need to glue on one side.
Fold the tube so that the tab is tucked inside. Run your finger along the full length of the join to make sure that it is glued firmly.
Squeeze the edges of the flattened tube to open it up to its square shape. The completed tube should be straight and accurate!