Imposition & Signature Calculator

Presses do not print pages, they print sheets — and a sheet gets folded, which means the pages on it are not in reading order. A sheet folded once carries four pages, folded twice carries eight, folded three times sixteen. That folded unit is a signature, and a book is a stack of them gathered in sequence.

This is why page counts want to be multiples of sixteen. A signature is bought and folded as a unit whether or not it is full, so a 100-page book printed in sixteens occupies seven signatures — 112 pages — and twelve of them are blank. You paid for the paper and the press time either way. Designing to 96 or 112 rather than trimming to 100 is where the saving is.

For saddle-stitched work the imposition follows from the nesting. Sheets sit inside each other, so the outermost carries the very first and very last pages, and each sheet inward moves one page in from each end. For sheet i counting outward, the outer side prints pages (N−2i+2) and (2i−1), and the inner side prints (2i) and (N−2i+1). Every sheet's pairs sum to N+1, which is the property that makes the numbers run in order once folded.

Creep is the physical consequence of that nesting. Each sheet is displaced toward the fore-edge by the thickness of everything wrapped around it, so the innermost spread sits furthest out and loses the most when the fore-edge is trimmed. On thick paper or a high page count that shift is millimetres, which is enough to cut into a folio or crop an image edge.

Sixteen is not a convention someone chose; it is how many times a sheet can usefully be folded. Each fold doubles the pages and halves the area, and after four folds the paper is too thick at the spine and the fibres start to crack along the crease. Thirty-two page signatures exist on thin papers, and eight is common on heavy stock, but sixteen is the number that works on most weights.

That physical limit propagated all the way up into how books are commissioned. Word counts, chapter lengths and picture sections are still negotiated against signature boundaries, because dropping from 240 pages to 224 removes a whole signature from the print bill while dropping to 236 removes nothing at all. An editor asking for eight pages to come out is usually asking for half a signature.

Digital printing weakened the rule without breaking it. Short-run digital work can be bound at any extent, so the penalty for an awkward page count is smaller — but as soon as a job is litho-printed in any quantity, the signature is back, and it is cheaper to know that at the layout stage than at the quote stage.