18mo
A book size octodecimo-sized pages.
18mo: a book format with 18 leaves per sheet
Octodecimo, abbreviated 18mo, is a book size produced by folding a single printed sheet into 18 leaves, or 36 pages. The sheet is folded three times to create this signature. The resulting page dimensions vary depending on the original sheet size, but a typical 18mo page measures roughly 4 by 6.5 inches, making it a compact format suitable for pocket-sized volumes, small reference books, and portable editions.
The 18mo format sits between smaller formats like 32mo and larger ones like 12mo in the traditional hierarchy of book sizes. It emerged as a practical compromise in hand-press printing when economy of paper and portability both mattered. Today it is less common than 8vo or 16mo, but still appears in specialty printing, religious texts, and antiquarian editions. The format requires careful imposition of type or film on the printing plate to ensure correct page sequence across all 18 leaves when the sheet is folded and cut.
Folding and Signature Structure
Creating an 18mo signature demands precision in both imposition and finishing. The printed sheet must be folded in a specific sequence, typically three parallel folds or a combination of parallel and right-angle folds. After folding, the signature contains 36 pages arranged in nested sections. If folding is misaligned or the cuts are uneven, pages will be misregistered, crooked, or incompletely severed, a defect called 'backed out' or 'picked' pages in trade parlance.
The 18mo format requires robust paper stock because each leaf is relatively thin compared to bulkier formats. Low-quality or highly absorbent paper may show through or tear along fold lines during cutting and gathering. Historically, 18mo volumes were often bound in stiff boards or leather to protect their small pages from wear.
While offset lithography and digital printing have largely replaced letterpress for trade books, 18mo remains relevant in limited-edition printing, hand-bound artist books, and restoration work. Understanding the imposition geometry for 18mo is essential for anyone preparing files for short-run specialty printers or working on historical book reconstruction.