In color reproduction, the word key (K) stands for depth or black. In subjective color mixing according to the CMY color model, the color black is produced by printing the three primary colorscyan (C), magenta (M), and yellow (Y). With this representation, the image depth is rendered in an improved manner. For this reason, black is printed in addition to improve the contours and increase the contrast. The corresponding color model is the CMYK color model.