How do you add page numbers in imposition? Upside-down pages and folios on a signature

In print, the page number is called a folio. Folios are not only for the reader: the binder and whoever checks the job use them to confirm that signatures were folded and gathered in the right order.

Some pages carry no folio: the cover, the title page, blank pages and full-page images often go unnumbered but still count. A page without a visible number still takes up a page.

When a signature is folded, the pages fall into reading order. To get there, some pages face head to head on the sheet: on a 16-page signature, half the pages are upside down on the press sheet.

If numbers are placed relative to the sheet, all facing the same way, they come out head-down on the pages that were upside down once the sheet is folded. A folio must always turn with its own page.

In a book, odd pages are on the right and even pages on the left. If the folio sits on the outside edge, it goes bottom right on odd pages and bottom left on even pages; if it is centred, it sits in the same place on both.

Keep the folio clear of the trim line. A small shift at the guillotine, or creep in saddle stitching, can clip it; a few millimetres inside the trim is safe.

Seeing the folded signature before going to plate is the most reliable check: with the pages in order, the folios should run 1, 2, 3 and all read the right way up.

To check by hand, fold and trim a proof sheet, then turn the pages and read the numbers. A missing, repeated or upside-down folio costs nothing if it is caught before the plates are made.