A printing press, especially one used in offset lithography, that allows the simultaneous printing on both sides of a sheet of paper in one pass through the press. There are two basic configurations of offset perfecting presses. In a convertible perfector, special transfer cylinders between successive printing units flip the paper over after it leaves the first impression cylinder, allowing the second unit to print on the reverse side of the sheet. Such presses have the advantage of being able to be used for single-side multi-color printing, simply by adjusting the transfer cylinders to keep them from flipping the sheet over. A second type of perfecting press, used primarily in web offset lithography, is called a blanket-to-blanket press, and utilizes one printing unit in which the impression cylinder is replaced by a second blanket cylinder directly below the first. As the sheet or paper web passes between the two blankets, images are printed on both sides at the same time. (See also Offset Lithography.)
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