The CMYK colour model is the subtractive four colour system that is usual in printing. CMYK represents the three fundamental colours cyan,magenta and yellow, as well as the key colour black. Each of the four colours has a value range from 0 % (unprinted surface) up to 100 % fulltone colour. As a rule the key colour K is not used directly for colour determination, but is used to darken some dark colour areas. A black colour share in the composite print also strengthens the contrast and depth. This is supplemented by the fact that while a black colour tone can be produced from cyan, magenta and yellow in theory, this does not always work in practice without light tints. This is due to the fact that the cyan, magenta and yellow pigments used in printing do not have any ideal absorption and remission characteristics.