Gutter margin

The gutter is the inner margin of a bound page, nearest the spine. Butterick's Practical Typography defines it: "A gutter margin is extra space on the inner edge of a page, used to account for a binding," and notes that in a two-sided document "the gutter margin will automatically alternate sides." A bound book does not open flat, so text set too close to the spine disappears into the curve; KDP's minimums rise with page count, from 0.375 in (9.6 mm) at 24 to 150 pages to 0.875 in (22.3 mm) at 701 to 828 pages, while the outside minimum stays at 0.25 in.

For the author, the gutter is why a print interior cannot reuse screen margins: minimums are floors, not targets, and a thick book held open at the minimum is a book the reader fights. Page count and trim size set it, bleed moves the outside minimum, and the walkthrough is how to create a print-ready PDF.