The front side of a printing medium is brought into surface contact with a transparent platen extending across the medium from side to side. The printing medium is of such type that enables characters printed on the reverse side thereof to be read from the front side thereof. Disposed on the reverse side of the printing medium is a printing block which travels parallel to the transparent platen for printing reversed characters on the reverse side of the printing medium at a position corresponding to the platen. Characters printed in reverse form on the reverse side of the printing medium appear in correct, readible form on the front side thereof and are visible through the transparent platen the instant they are printed.