signature
A group of four (or a multiple of four) sheets printed such that, when folded, they become a section of a book.
signature: folded sheets that form a book section
In bookbinding, a signature is a nested stack of printed sheets that, when folded in half, creates a bound section or gathering of a finished book. If you unfold a signature, you see it is actually a larger sheet printed on both sides with multiple pages arranged in a specific sequence. Once folded and nested with other signatures, the page numbers align correctly and the sections stack to form the book block before casing.
Signatures come in multiples of four pages because a single sheet folded once creates four pages: two on the front half and two on the back half. A signature may contain 4, 8, 16, or 32 pages depending on how many times the sheet is folded. Most book production uses 16-page signatures (also called 16mo or sixteenmo), which balance efficient use of large press sheets with manageable handling during binding. The term itself comes from the old practice of printing a small letter or symbol in the margin of each signature to help the binder collate them in the correct order.
Page imposition, the arrangement of pages on the printing sheet before folding, is critical to signature production. Pages must be positioned so they fall in correct numerical order and orientation after folding. Any error in imposition makes the entire signature unusable. Printers use specialized software to calculate impositions, and the sheet is run through the press twice (work-and-turn or work-and-tumble methods) to print all pages efficiently on standard press sizes.
In offset lithography, signatures are printed on large sheets, typically 25 by 38 inches or larger, then folded on a folding machine. The creasing and folding must be precise; wrinkles or misalignment waste the entire signature. In digital printing workflows, signatures may be smaller and printed from different masters, but the nesting and collation process remains the same.
Signatures represent a fixed production unit in bookmaking. A book's page count must be a multiple of the signature size, or blank pages must be added. A 200-page book using 16-page signatures requires exactly 12.5 signatures, so 224 pages must be printed. This constraint shapes book design and explains why not all books end on exact hundreds or thousands of pages.