Pages are printed many at a time
Offset presses print large sheets — each carrying many book pages on each side. After printing, the sheet is folded several times into a compact bundle called a signature. The signatures are gathered in order, sewn or glued, and trimmed on three sides to free the pages.
Folds and formats
The number of folds determines how many pages a signature holds, and historically gave book formats their names:
| Folds | Leaves | Pages | Traditional name |
|---|---|---|---|
| 1 | 2 | 4 | Folio |
| 2 | 4 | 8 | Quarto (4to) |
| 3 | 8 | 16 | Octavo (8vo) |
| 4 | 16 | 32 | Sextodecimo (16mo) |
Because the original sheet sizes varied, "octavo" once described a size range as well as a folding method — roughly the size of a modern hardcover novel.
What imposition does
Imposition is arranging pages on the sheet so they come out in order after folding. Some pages end up upside down on the flat sheet, and pages that seem unrelated sit side by side. A simple check: in a 16-page signature, pages printed next to each other across the spine fold always add up to 17 (16 and 1, 4 and 13). Our 16-page diagram shows both sides of the sheet.
Why page counts come in multiples
An offset book is a whole number of signatures, so its page count is planned in multiples of 16 or 32 (sometimes with an 8-page signature at the end). A 300-page text becomes 304 pages — nineteen 16-page signatures — with four blank or extra pages. That is why older books often end with blank leaves or advertisements. Print-on-demand books are printed differently and only need an even page count.
Saddle-stitched booklets
Magazines, programmes and zines are made by nesting folded sheets and stapling through the fold. Each sheet carries four pages, so the page count must be a multiple of four. The outer sheet holds the first two and last two pages. See the page order for any booklet size or use the booklet calculator to print your own.
Grain and creep
Paper fibres have a direction (grain). Printers arrange sheets so the grain runs parallel to the spine of the finished book, which helps pages turn smoothly and lie flat. In thick saddle-stitched booklets, inner sheets push outward at the fore-edge (creep), which imposition software compensates for by shifting inner pages slightly toward the spine.
Make a folding dummy
Take a sheet of paper, fold it the way the signature will be folded, number the pages through the folded bundle, then open it out. You have just made an imposition — the same method printers used for centuries.