The offset lithographic process works by first transferring an image photographically to thin metal, paper, or plastic printing plates. Unlike other forms of printing, in offset lithography the image on the printing plate is not recessed or raised. Rollers apply oil-based ink and water to the plates. Since oil and water don't mix, the oil-based ink won't adhere to the non-image areas. Only the inked image portion is then transferred to a rubber blanket (cylinder) that then transfers the image onto the paper as it passes between it and another cylinder beneath the paper.
The term offset refers to the fact that the image isn't printed directly to the paper from the plates, but is offset or transferred to another surface that then makes contact with the paper.
The printing plates used depends on the type of press, the printing method, and quantity of the print run. A plate is prepared for each color used, or four plates in the case of 4-color (CMYK) process printing. In general, metal plates are more expensive but last longer and have greater accuracy. Paper plates are usually more suitable for shorter runs without close or touching colors.
