PPI (pages per inch).

PPI is what turns a page count into a physical object: spine width, board size, carton fit, shipping band. It is a property of the paper rather than of the book, and the spread is wide. The binder's bulk tables run from 1,454 pages per inch at a caliper of 5.5 thousandths down to 160 at 50.0 (Etherington and Roberts, bulk equivalents). Print-on-demand platforms bury the figure inside a spine formula instead of quoting it, so KDP multiplies page count by 0.0025 inch for cream paper and 0.002252 inch for white, allows 0.0625 inch of variance either side of the fold lines and calls a spine shifted more than 0.0125 inch defective (KDP cover specification). Those two multipliers are 400 and roughly 444 pages to the inch, and a cover built on the wrong one puts the spine artwork on the front. On a print-on-demand title that is not one bad run to reprint, it is every copy sold since the file went live. The same three letters on an image spec mean pixels per inch, which has its own entry.