Bleed is the amount of artwork that falls outside of the printing bounding box, or outside the crop area and trim marks. You can include bleed in your artwork as a margin of error—to ensure that the ink is still printed to the edge of the page after the page is trimmed or that an image can be stripped into a keyline in a document. Once you create the artwork that extends into the bleed, you can use Illustrator to specify the extent of the bleed. Increasing the bleed makes Illustrator print more of the artwork that is located beyond the trim marks. The trim marks still define the same size printing bounding box, however.

The size of the bleed you use depends on its purpose. A press bleed (that is, an image that bleeds off the edge of the printed sheet) should be at least 18 points. If the bleed is to ensure that an image fits a keyline, it needs to be no more than 2 or 3 points. Your print shop can advise you on the size of the bleed necessary for your particular job.