Compensating for press misregistration by adjusting the artwork of a publication. Modern printing equipment is very accurate; however, since paper stretches, curls, or twists as it passes through the press, it is impossible to print with perfect registration. Misregistration is most visible when there is a visible gap between two overlapping colors and the paper shows through or when two color overlap to produce a third color. To compensate for this, the artwork is modified so that the lighter color is spread, or made slightly larger, or the darker color is choked, or made slightly smaller. If this is done properly, the registration can be slightly off without being noticable.
See Scotts Valley Printing's Trapping FAQ for more information.