Key (K)
The black ink in four-colour printing, the K of CMYK.
Also: key plate, black plate
In printing, the key plate is the one that carries the main detail and outline of an image, to which the other colours are aligned, or keyed. It is almost always printed in black. That is the usual explanation for the letter K in CMYK. It is also said that K was chosen to avoid confusion with B for blue. The black plate sharpens detail, deepens shadows, prints text and saves on the more costly coloured inks. This sense of key is separate from the key colour of a design palette.