design_trappingIf you print one object next to another their edges need to match up or you’ll get little white lines between them. Given the sorts of detail which we need, and knowing the tolerances of the presses we use, we have come up with techniques to fool they eye into not noticing the very slight mis-registration which is almost inevitable in any printing job. In the prepress world this is called trapping. The yellow circle at the lower right is trapped — the image has been enlarged so that even if it when printed off register your eye won’t be aware of the problem.

This page from graphicdesign.spokanefalls gives a good description for how to prepare artwork to avoid mis-registration.

On press, trapping simply means printing one color of ink on top of another.