Add hidden gutter page numbers
A hidden folio is a small number set close to the bound edge, out of sight once the book is assembled. It is the standard compromise for books where a visible number would spoil the page.
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Where the number lands
Roughly 5 to 6 mm in from the binding edge, near the bottom, at about six points. Odd and even pages mirror, so with the binding edge set correctly the number always lands on the gutter side. In a perfect-bound book it disappears into the glue margin; in a saddle-stitched one it sits beside the fold where nobody looks.
Whether it is accepted is up to the printer
Plenty of Japanese printers explicitly allow hidden folios, but some insist on a visible number on every body page. Placing the number where trimming removes it entirely defeats the purpose and can get a file rejected, so keep it about 5 mm inside the trim. Bleed, trim marks and paper thickness differ from printer to printer — always check the spec sheet of the shop you are submitting to.
When to use the visible kind instead
Prose and reference books are kinder to readers with an outer-bottom folio they can actually find. Hidden folios suit art books, photo books and full-bleed colour comics. To mix both in one book, process the two page ranges separately and merge the results.
Frequently asked questions
What size should the number be?
Five to seven points is normal. Smaller than that and press gain can fill the digits in until nobody can read them, so the default here is six.
Will trimming cut it off?
Not at 5 to 6 mm inside the binding edge. Perfect binding hides it under the glue, but it stays in the file, which is what the printer checks.
Does it go on the cover too?
Only if you let it. Covers are usually submitted as a separate file, so process the body pages alone, or list the cover in the skip field.