What is perfect binding?
Perfect binding glues the pages of a book or catalog into a wraparound cover at the spine, giving a square, flat spine that can carry printed text, like a paperback.
What it means for your file
The pages are gathered, roughened along the spine edge and glued into the cover, so most printers need only an even page count. The cover is one flat file, with the back, spine and front side by side, and its spine width depends on the page count and the paper’s thickness.
Thin books don’t have room for spine text: KDP, for one, only prints it on books of more than 79 pages. For stapled booklets, see saddle stitch.
An example
A 6 × 9 in paperback of 240 pages on Amazon KDP’s white paper has a 0.540 in spine (240 × 0.002252 in). Its cover file is one flat sheet, back cover, spine and front cover side by side with 0.125 in of bleed all round: 12.79 × 9.25 in.
A common mistake
Setting text too close to the spine edge of the inside pages. A glued book doesn’t open flat, and the binding takes a little of each page, so text near the inner margin disappears into the fold. Give the inside (gutter) margin more room than the outside one, more again in a thick book, and check the printer’s minimum.
How to fix it
If your printer sent the file back saying “The spine width doesn’t match the page count,” the fix page has the step-by-step way to do it yourself, and a fixed price if you’d rather have it done.
Where else it comes up
All print terms in the glossary, or run your file through the free print-ready checker.
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