The Y of CMYK enters the press. (Cyan, Magenta, Yellow, Key black. The black plate is traditionally used for the key to align the plates. In principle, with a subtractive process such as printing, one can get black by combining C, M, & Y. But the addition of a separate black plate saves ink, reduces drying times, and allows for details in black typefaces & art. Ref: Wikipedia, 11/2010.)