vigesimo-quarto
Formed of sheets folded so as to make twenty-four leaves.
vigesimo-quarto: a book folded into 24 leaves per signature
Vigesimo-quarto, or 24mo, is a book format in which a single large printed sheet is folded multiple times to create 24 leaves, or 48 pages. Each leaf is one twenty-fourth the size of the original sheet. This format emerged from the practical economics of letterpress printing, where minimizing waste and maximizing output per sheet mattered intensely to printers and publishers.
The folding sequence that produces vigesimo-quarto requires precision. A sheet must be folded four times in specific directions: first into half, then quarters, then eighths, then sixteenths, and finally into twenty-fourths. Alternatively, a sheet folded in half three times produces eight leaves (16mo), and a fourth fold creates the 24 leaves. After folding, the signature is collated with others and sewn or glued into the binding. Trimming removes the folded edges so each leaf becomes two separate pages.
Vigesimo-quarto became common for pocket-sized religious texts, almanacs, and practical handbooks where compact dimensions were valued. Paper economies made it attractive: a sheet that would be cut for 4to or 8vo format could yield double or triple the page count as 24mo. The trade-off was readability. At 24mo size, typically 4 to 5 inches wide, type had to be small; leading between lines was reduced; margins nearly disappeared. Only books intended for reference or devotional use, where readers expected density over comfort, were regularly published in this format.
The vigesimo-quarto format declined sharply after the mid-twentieth century. Industrial printing shifted toward web presses and standard sheet sizes; the labor and skill required to fold and collate signatures by hand became uneconomical. Paperback binding and new trimming machinery made intermediate formats like 12mo and 18mo more efficient. Today, vigesimo-quarto is mainly encountered in antiquarian books, theological volumes, and historical pocket editions.
The term itself follows the Latin nomenclature used in bibliography and printing: vigesimo means twentieth, quarto means fourth. The full name indicates that this is the twenty-fourth part of a sheet, though the ordinal numbering convention is old and inconsistently applied across languages and print traditions. In English, the format is often abbreviated simply as 24mo to avoid the awkward Latin adjective.