Offset lithography is an analog printing process still the most common way to produce printed materials on paper today. Offset refers to the technique in which the inked image is transferred (or 'offset') from a plate to a rubber blanket, then to the paper surface. Lithography is based on the repulsion of oil and water. Offset lithography uses oil-based inks. The plate on which the image is carried obtains ink from the ink rollers, while the non-printing area attracts a water-based film (called 'fountain solution'), keeping the non-printing areas ink-free. Offset lithography prints images in four colours: Cyan, Magenta, Yellow and Black (CMYK). Pre-press - specifically colour separations, the converting of RGB images to CMYK - is an essential part of the offset lithography process. Offset lithography is optimal for economically producing large volumes of high quality prints. In terms of quality, offset lithography is regarded as the benchmark, with digital printing processes ranked in accordance with their ability to achieve offset like results. Today, with continued improvements in digital printing, the benchmark for quality is shifting, with digital being at least equal to litho, or considered on its own terms for a specific look or finish that is not achievable with offset lithography.