Although it is theoretically possible to achieve true black by combining cyan, magenta and yellow, it does not work in real world situations. Because inks do not combine in the same way that light does cyan, magenta and yellow when mixed together yield a deep purple-brown that is far from the expected black. Therefore, printers use four inks (CMY and black-K). Using K allows a true black as opposed to the purple/brown that is yielded by the combination of CMY. There are more complex reasons for using black in addition to CMY.
Offset presses are used to cheaply reproduce in high volume. A classic example of an offset press is a newspaper press. Ink is put onto paper via custom made plates that are somewhat analogous to rubber stamps. When an offset press prints something in color, cyan, magenta, yellow and black are used. All the inks are applied separately so that in a sense the color image is printed four times. Once for cyan, once for magenta, once for yellow and finally once for black.
Since we can think of the image as being printed four times, there need to be four plates made; one for each CMYK component. The examples below show a photo and it's separated CMYK components. If this image were to be printed on an offset press, the plates for each color would look much like the color separations below.
Full color CMYK Image
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| Cyan Seperation | Magenta Seperation |
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| Yellow Seperation | Black Seperation |
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