When full colour products are
printed each of the four printing colours are added separately.
C M Y
The yellow is printed onto the paper, then the cyan and so on...
Each colour is added to the paper in a different part of the machine.
The paper travels from the yellow printer to the yellow and so on...
It is important that each colour is lined up exactly with the previous
Registration marks are used for this...
They are usually a cross with a circle around it. They are printed
4 times, one in each colour over the top of each other...
If the colours are lined up accurately you should only see a black
registration mark. If one of the colours are out of line you will
see that colour registration mark printed slightly to the side of
the black registration mark...
If the colours are out of line the final image will appear blurred.
When printing a lot of something the registration marks are checked
every so often to avoid reproducing a lot of errors.