A “bleed” could include bars, graphics, or rules with ink visible at its edges extending outside this trim zone of pages in bound books.
A “bleed” refers to printing extending to the paper’s edge after trimming, usually when an image stretches across it. Bleeds are necessary when your design or illustration extends all the way across, as, without one, your printer would need to print on an even larger sheet and trim down later, creating an undesirable white border around your design.
To avoid this issue, your design must extend past the edge of your document to form a “bleed.” For instance, when designing for an 8.5×11 flyer, you would open it by an extra quarter-inch on both sides – this extra space forms your “bleed.” Once printed, it will be reduced back to fit 8.5″x 11″, producing a design that runs across.
Additionally, in addition to extending your design beyond the edges of the document, bleed margins must also be added for text elements close to the edge of the page. This ensures that any slight variations can result in text being cut off prematurely if printed and cut too close during the printing and trimming processes. By including bleed margins with text elements close to the edges of pages, you ensure text stays within safe zones without getting cut off accidentally.
Under normal circumstances, text elements within 0.5 inches from the edge of a page require an extra 0.125″ bleed margin—for instance, if a component were only 0.25″, you would extend it by 0.375″, thus creating a total margin length of 0.375″.
While bleed margins may not be necessary for every document, they’re recommended when wanting to play it safe. Also, printers or publications with specific requirements might ask that they include them for publication.
As stated, bleed is an extension of your design or image beyond the edge of your document, and bleed margins refer to additional margins you add on text elements that come close to touching this edge of the page.