Definition
Spine
The spine is the bound edge of the book, carrying the title, author and imprint. Its width is calculated from page count and paper thickness, so it can only be finalised after the interior is typeset.
Spine width is page count multiplied by the caliper of the chosen paper, and print-on-demand platforms publish the formula for each stock. That dependency is the reason a cover cannot be finished before the interior: add ten pages and the spine grows, the wrap shifts, and centred spine type is no longer centred. Below roughly 100–130 pages most platforms will not print spine text at all, because the tolerance of the trim exceeds the space available.
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