Because the transfer was made directly to the substrate, it was necessary to generate a mirror image on the plate (wrong reading is often used but mirror image is more precise since there can be many different wrong reading images possible, but only one mirror image). This was changed by the invention of the ìOffsetî press attributed to Ira Rubel in 1904. In this process, a set of three rotating cylinders is used and the image is first transferred from the plate (on a plate cylinder) to a rubber blanket (on a blanket cylinder) from which it transfers (offsets) to the substrate.
A schematic of the cylinder arrangement of an AB Dick duplicator press is shown at right. On this we see the master, or plate, cylinder, the blanket cylinder and the impression cylinder.
The substrate passes between the blanket cylinder and the impression cylinder. The other rollers are part of the inking and dampening systems, which we will return to later.