A proof is a typeset version of copy or a manuscript page. They often contain typos introduced through both human error and bugs in programming code. Traditionally, a proofreader looks at an increment of text on the copy and then compares it to the corresponding typeset increment, and then marks any errors (sometimes called line edits) using standard proofreaders' marks. Thus, unlike copy editing, proofreading's defining procedure is to work directly with two sets of information at the same time. Proofs are then returned to the typesetter or graphic artist for correction. Correction-cycle proofs will typically have one descriptive term, such as bounce, bump, or revise unique to the department or organization and used for clarity to the strict exclusion of any other. It is a common practice for all such corrections, no matter how slight, to be sent again to a proofreader to be checked and initialed, establishing the principle of consistent accuracy for proofreaders.