What it means
The gutter is the space at the inside edge of a page, next to the spine — technically the pair of inner margins on a spread, though the word is often used for the extra allowance added to them. It matters because a bound book does not open flat: in a perfect-bound paperback or a casewrap hardcover, some of the inner margin curves down into the binding and is lost to the eye.
The thicker the book, the more it loses, so gutter allowances scale with page count. A widely used schedule adds about 0.375 in for books up to 150 pages, 0.5 in up to 300, 0.625 in up to 500, 0.75 in up to 700 and 0.875 in beyond that.
Too small a gutter makes readers crack the spine to see the text; too large a one wastes paper and pushes the text block outward. Lie-flat bindings such as coil sidestep the problem by opening fully.
In Bindery
Bindery’s free watermarked proof shows the typeset spread at its real trim, so the gutter can be judged before printing; for books that must open flat, its coil-bound preset holds 2–470 pages.
Make the book, not the checklist
Describe a book or upload a manuscript. Bindery writes with you, typesets in LaTeX, sizes the spine from the real page count and produces printer-validated files — the watermarked proof is always free.
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