A sheet of paper does not come off the press with the pages in reading order. A book is printed on large sheets that carry many pages at once, and those pages are positioned so that when the sheet is folded and the folds are trimmed, the numbers run 1, 2, 3 down the block. The arrangement is called imposition, and it is one of the first things prepress settles before a plate is made.
Why the order changes
Print one page per sheet and the job is simple and slow. Fold the sheet into a signature and you can print 16, 32 or 64 pages on each side in a single pass. The pages are laid out on the flat in the positions they will occupy after folding, not in the order a reader sees. A 16-page signature, for example, stands for a sheet folded four times. Page 1 sits beside page 16, page 2 beside page 15, and so on, because each pair ends up facing each other once the sheet is folded and the folds are cut.
Folding schemes
The number of folds decides the scheme and the page count. A sheet folded once makes a folio of four pages. Fold it again for a quarto of eight, again for an octavo of 16, and once more for a sixteenmo of 32. The printer picks the scheme to suit the finished size, the press and the weight of the paper. Heavier stock folds less cleanly, so a bulky paper often forces a smaller signature.
Work and turn, work and tumble
For short runs there is a shortcut. Instead of two plates, one for each side, a single plate prints the same pages twice. The sheet is printed, then turned over and run through again. Turned on the long edge, the method is called work and turn. Turned on the short edge, it is work and tumble. Both save a plate and a setup, and both demand that the gripper margin and the margins of the layout are planned around the flip.
Where the fold pushes out
When a stack of folded signatures is gathered, the inner pages push out at the fore edge because of the thickness of the paper. That creep, sometimes called shingling, has to be compensated, or the inner pages will be trimmed too far and lose their margin. Imposition software shifts the pages a little towards the spine in proportion to the paper thickness, and the amount grows with page count.
What the plate carries besides pages
A modern imposition is not only pages. The flat carries fold marks, cut marks, colour bars, registration targets and the gripper margin that the press needs. On a CTP system several jobs can share one plate, a practice called ganging, so long as the colour and the stock match.
Where it goes wrong
The page count has to divide by the signature. If it does not, the printer adds blank pages or a smaller final signature, which is why a book is often 160 or 168 pages and rarely 163. The second trap is a page placed in the wrong position, so that a spread reads 5 then 8 after folding. The third is the gripper: text placed inside the margin the press cannot print will be clipped on the sheet, and no amount of careful trimming will bring it back.
Imposition is invisible in the finished book. When it is right, page 1 follows the cover and page 200 follows page 199, and nothing looks arranged at all. As book printing continues to lead in production quality and innovation, the industry sets new benchmarks for print manufacturing worldwide.
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