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Nothing is perfect, not even obscenely expensive printing presses. When your print job is flying through those presses, each color being added one at a time, the paper may shift slightly. Depending on the press, this could be an offset of anywhere between .003 and .0625 inches (.2 to 4.5 points). If two colors abut each other on your artwork and this shift occurs, then the two colors may be moved apart slightly, resulting in a white “unprinted” space. It may seem like a 3/1,000-of-an-inch space would look like a small crack in a large sidewalk, but I assure you, it could easily appear to be a chasm. What can you do? Fill in these potential chasms with traps and overprints.