Ink is applied to the metal plates via series of rollers and then, via another series of rollers, to the paper. Because the plates never actually touch the paper itself the printing operation is referred to as 'offset'.
The first is process, or CMYK printing. CMYK refers to the four ink colours used (cyan, magenta, yellow and black, known as in print parlance as key — hence K).
By using these four colours it is possible to print almost every colour of the spectrum. It is suitable for the vast majority of colour printing and since the press is always loaded with these four colours of inks, it is surprisingly inexpensive.
The other common use of offset litho machines is called spot colour printing. This is used where precise colour matches are needed.
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