How to make the cut
Considering how important bleeds are to print production, their use and necessity are seldom understood. Every designer has been nagged to add a bleed to their file at one point or another, but they aren't often told why. What is a bleed? When do you need to include them, and why is it so important to include them at all?
Remember arts and crafts when you were little? When coloring a sky or ocean, you may have colored directly off the page, running your crayon or marker off the actual drawing so that the color got over every edge. Or maybe you were coloring and cutting out a shape for another craft, and rather than cut, then color, you colored a giant swatch to cut the shape out of. This is what it means to bleed graphics.
A bleed is additional graphics or color that is trimmed off of the final print product. Bleeds ensure that a slight misregister during trimming doesn't result in a white edge at the cut area. The print standard for bleeds is usually .125 in, but it does vary by printer and project.
When do you need a bleed? A bleed is necessary anytime a project uses a photo, color block, or any graphic that runs to the edge of the trimmed page. If the entire edge of your project is white, no bleed is required. Bleeds are important regardless of the print process, but are especially critical for offset lithography.
Now for the why: humans and the machines we use are not perfect, and even the most meticulously executed job will have some variation. When a project has graphics that go to the edge of the trimmed sheet, this variation typically results in a hairline bit of white along the edge. That hairline error is only perceivable to the naked eye when it is highlighted by a break in a solid graphic. Bleeds act as insurance that a misregister won't show a white edge.
How do you apply bleeds? Every professional graphic production software will have easy to use bleed capabilities. Document settings, page settings, etc. will have the option to set a bleed. Typically once a bleed is added to a setting, the page or artboard dimensions will not change, but a guide will be added showing the bleed edge. Anything within this bleed guide will be trimmed off; however, if exported properly, the document will maintain the bled graphics. Be sure to check your export options to include the bleed if you are exporting to a pdf. It is possible to export a bleed without adjusting document settings, but most software will default to the document settings. Simply telling your document to export with a bleed will not work unless the graphic comes off of the page or artboard. Remember that you only need to bleed graphics when they go to the edge of the page.