Gutter.

An additional margin on the bind side of interior pages. Lulu describes it as compensation for the part of the page the binding makes unusable, and says interiors under 60 pages do not normally need one while files with a higher page count should consider it. Lulu's table adds nothing below 60 pages, 0.125 in for 61 to 150, 0.5 in for 151 to 400, 0.625 in for 400 to 600 and 0.75 in above 600, and recommends interior margins from 0.5 in to 1.25 in. KDP sets the inside (gutter) margin by page count instead: 0.375 in for 24 to 150 pages, 0.5 in for 151 to 300, 0.625 in for 301 to 500, 0.75 in for 501 to 700 and 0.875 in for 701 to 828. The two tables give different values for the same page count, so the gutter belongs to the platform and the final page count, and a book moved between platforms needs it reset. A manuscript laid out with equal margins on all sides is the one whose text runs into the fold.