In depth
In saddle stitching, the folded sheets are stacked over a saddle-shaped support, the stack is straddled along the spine, and two (occasionally three) wire staples are driven through the spine to bind the pages. The finished booklet opens flat, which makes it the preferred binding for catalogues, manuals, and magazines up to about 64 pages.
Page count must be a multiple of four — every sheet folded in half creates four pages — and the practical upper limit is set by what the wire stapler can handle, plus the visible creep as inner pages drift outward at the spine. For thicker books, perfect binding or PUR binding replaces saddle stitching.
Imposition for saddle stitch is specific: pages are arranged in "reader spreads" that, when folded and gathered, sequence correctly. Pages 1 and the back cover share the outermost sheet; the middle pages share the innermost sheet. Imposition software handles this automatically; doing it by hand is a recipe for errors.