alignment crosses
Alignment crosses are the small crosses printed outside the finished page so that a printing works can lay its plates exactly over one another.
In more detail
Each ink goes down in a pass of its own, and the sheet has to come back to the very same place each time. A cross printed by every plate at the same spot is how that is checked: if the crosses fall exactly on one another, the inks agree; if one of them sits a hair to the side, everything printed in that ink does too.
They stand outside the cut, on the part of the sheet the blade takes away, so nothing of them shows on the finished page. They answer a different question from the trim marks beside them: those say where the blade goes, these say whether the inks are on top of one another.
What it does on a real document
Drawing them in black does not do the job. Black is one plate among several: a cross laid in black comes out on the black plate and on no other, and lines up with nothing. What is wanted is a mark that every plate carries.
There are two ways to ask for that. One ink name is set aside for all the inks at once: a mark drawn in that ink is laid on every plate, whatever inks the job runs on. That is what the two-ink example on this site uses for its four crosses. The other way is to ask for the four usual inks — cyan, magenta, yellow and black — each at full strength, which comes to the same thing on a job printed in those four.
Where it comes from, and why
They are as old as printing a sheet in more than one colour, and the trade's word for what they check is register. A sheet out of register shows it plainly: the letters carry a coloured fringe on one side, and a photograph looks faintly doubled, as though it had been printed twice a hair apart.
The operator watches them through the run and brings the press back when it drifts. That is why they are printed at all, out on the waste the blade takes away.
See It on a Finished Document
Inks on Plates of Their Own The page that goes further on this word