Duodecimo, sometimes voiced twelvemo (and occasionally written in the same way, or as 12mo or 12°), is a small size of book between octavo and sixteenmo or sextodecimo. In Moby-Dick Herman Melville assigned the dolphins and porpoises to his classification of duodecimo whales: a 12mo book was a small relatively narrow object. Its smallness meant that the paper savings sometimes made the extra labor of producing it worthwhile.

  • A folio has been folded once, (resulting in 4 pages on 2 leaves)
  • a quarto twice (4 leaves, 8 pages)
  • an octavo thrice (8 leaves, 16 pages)
  • a sextodecimo four times (16 leaves, 32 pages)
  • there’s even a tricesimo-secundo, 32mo, folded eight times (32 leaves, 64 pages)

Keep you eye on the leaves (each one consisting of two sides, thus two pages) and the system makes sense. A duodecimo will have 12 leaves, 24 pages — something which can’t be achieved by just folding a single sheet of paper. (I’m not talking about a web press here, just sheet-fed.) There’s also a vicesimo-quarto, a 24mo: just think twice the 12mo structure described below. These names, while they may have actually been used by real people in the dim and distant, are now in the exclusive possession of the bibliophilic community. The only interaction we have with them is when we see reference to Shakespearen Folios and Quartos, and in the quaint proclivity of Brits to talk about Demy octavo when they mean what we’d call 5½” x  8½”.

Keith Houston helpfully provides a layout for duodecimo in The Book. His version displays both sides of the sheet shown side by side. The watermark he’s added shows this, as does following the page numbers.

To make your duodecimo book, you’d print up your 24 page sheet (12 to view), cut away the eight pages at the foot, inserting them after folding into the middle of the remaining 16-page section to give you the full 24-page section.

Once upon a time these terms defined trim sizes. You’d combine the name of the sheet size with the number of folds/leaves, to come up with names like Royal octavo, Crown quarto, Demy octavo. Nowadays we’ve thrown in the towel and resorted to inches or millimeters. So although a duodecimo implied a small, narrow book, just how small it would be would depend on the sheet you started with. A Royal duodecimo would end up as a 4⅞” x 8⅛” book while a Crown duodecimo would trim down to 3⅝” x  6⅜”. Actually I don’t think Royal or Crown sheets were much used for duodecimo — think of them as being here merely as examples.

Photo: Slow Industries

Here is a book where format matches content. Picking up on Melville’s whale classification, this duodecimo volume examines the Duodecimo Whales. The publishers, Slow Industries also have larger volumes covering the Folio Whales and the Octavo Whales.