a method by which stacks of printed sheets can be formed and , at the same time , the risk of soiling of the stacked sheets can be reduced to the maximum extent . to this end a printing machine is proposed , in particular a sheet - fed printing machine , into which unprinted and intermediate sheets are alternatively feed , wherein the printing is registered such that the printing occurs only on the unprinted sheets and not on the intermediate sheets . in this way , the intermediate sheets prevent damage to the print on the printed sheets .