Key - What does key mean?
One of the four colours used in four colour process printing; this colour is actually black. Black is referred to as "Key" because early four colour process printing used plates (one for each colour) to transfer ink to the correct position on a substrate; while the cyan, magenta, and yellow plates were used to add colour, a fourth key plate was required to add detail (such as lines and contrast) and black was the most common colour used on this plate.
It is necessary to include black as a fourth colour for a number of reasons. While combining the other three colours should produce a perfect black that absorbs all visible light, the actual result is a dark muddy colour (sometimes known as composite black or process black). If a very deep black is required, a coloured or grey bedding mix of CMY is laid down first and a full black layer is added on top; this is known as rich black. Combining the three primary colours usually creates a slight blur and so, as the detailed elements of an image (such as text and outlines) are usually printed in black, using a separate black colour means that it is possible to produce finer details. When inks are being used, layering three colours in one place tends to soak paper substrates, which increases the likelihood of bleed and can prevent the ink from drying properly, if at all. Using black ink is also less expensive than using the corresponding amounts of CMY to create black.Go Back to Glossary