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You’ve probably seen something like this in a book at one point or another.  This seemingly contradictory statement is there for a good reason: it’s there to prove that the printing process worked, and that the customer isn’t getting a botched print job.

Books are composed of signatures: one large sheet folded to compose several pages.  Three edges are then lopped off, producing a book bound only on one edge.

Sheets are easy to fold in powers of two: 4, 8, 16, and even 32 page signatures are reasonably common.  Presumably, most writers don’t sit down with the goal of writing a book that will fit exactly.  This is one reason why many books have ads, pages for notes, or simply blank pages at the back (or the front, for that matter).  It’s cheaper for the printer to leave the extra paper in than it is to rip them out, and it gives a nicer finish.