Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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The plan of arrangement of the pages of type in a chase so that they will read consecutively when the printed sheet is folded. It is a term which means literally "in position," and originated in letterpress printing.

There are four standard methods of imposition: 1) sheetwise; 2) work-and-turn; 3) work-and-tumble; and 4) work-and-twist. The purpose of having different schemes is to obtain maximum economy in printing.

Sheetwise imposition, also known as "work-and-back," "front-and-back," or "print-and-back," requires two image carriers per color, one each for the front and back of the sheet. This method is used for printing jobs where the number of pages to be printed on each side of the sheet is large enough to utilize the full capacity of the press.

Work-and-turn imposition, also known as "print-and-turn," is used where it is practical to print both sides of the sheet from a single image carrier per color. It is frequently used, and requires relatively large presses because the sheet will carry two complete units, each occupying one-half of the sheet. The sheet is either slit during backup or is cut apart after printing. Workand-turn imposition cuts the number of sheets to be printed by 50%, but it generally requires larger and more expensive presses than sheetwise imposition.

Work-and-tumble imposition, also known as "print-and-tumble," "workand-roll," or "work-and-flop," is used less frequently than either of the two previous methods. It is selected when a work-and-turn form cannot be used, or when two sheetwise forms would have to be run on a sheet of difficult dimensions. This method needs only one image carrier for printing both sides of the sheet, and in this respect it is similar to work-and-turn.

Work-and-twist imposition, also known as "work-and-twirl," differs fundamentally from all three previous schemes. While the first three are methods designed to produce sheets printed on both sides, work-and-twist imposition solves problems pertaining to one-side printing, e.g., printing blank rule and tabular forms, with cross rules in one section and vertical rules in the other. In this method, two separate sections, or divisions, of an individual form are imposed and locked up together in such a manner that they may be printed side by side on a double-sized sheet in one impression. After completion of the run, the already printed paper is repositioned face up for the second printing from the same image carrier. Work-and-twist imposition is actually rarely used, as it requires nearly perfectly square stock for execution. (234 , 287 , 289 , 316 , 320 , 339 )

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