A proof is a typeset version of copy or a manuscript page. They often contain typos introduced through human error. 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. Unlike copy editing, the defining procedure of a proofreading service is to work directly with two sets of information at the same time. Proofs are then returned to the typesetter 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 initiated, thus establishing the principle of higher responsibility for proofreaders as compared to their typesetters or artists.