Bleed is a printing term referring to photos or color that extends to the edge of a page in one or more directions. Though all colors are registered on a single sheet, each sheet can enter the press at slightly different positions, varying from hundreds to tenths of an inch. As printed paper is stacked, folded, or duplex printed, objects close to edges reveal variance.
Printing ink directly to the edge of paper usually results in ink transferring to the underside. So for bleeds, background is extended beyond the desired edge (usually 1/8" in each direction), printed on a larger sheet, and then trimmed to the desired size. A full-bleed letter size sheet (8.5" x 11") must therefore be printed on 9" x 12" or 11" x 17" paper and trimmed. A tabloid (11" x 17") page with bleed is printed 12" x 18" before trimming. Pricing takes into account the larger paper and time to trim. Obviously, printing with a good margin is most often more economical than printing with bleed.
There are some exceptions where bleed does not increase cost. Gang run offset printing is generally standardized to assume each job has a bleed. With all customers sharing the cost, bleed is included in the low price.
Large-format inkjet printing supports edge-to-edge printing on certain papers and widths. Glossy photo paper (often 42" wide), supports edge-to-edge printing. In other instances, a bleed may not be required for straight-cut posters even if color extends to the edge because each sheet is manually cut. Exceptions occur when banners are hemmed, or feed into retractor bases. Additional image or background color may extend into these areas.