As Senefelder refined the lithographic process, he discovered that an image printed on paper could also be transferred (or offset) to another stone to make as many identical copies as desired. The first offset machine put to general practical use had the usual reciprocating bed carrying the litho stone but in addition it had two cylinders, one above the other. The lower cylinder was covered with a specially prepared card on which the print was taken from the stone. The sheet of metal then passed between the two cylinders and received the impression on the underside, by transfer from the card. After many experiments the card was replaced with a rubber-covered cotton sheet, or blanket.