What does duodecimo mean?
A duodecimo is one of the traditional book sizes used by printers and bibliographers to describe the format of a volume. It results from folding a full printer's sheet into twelve leaves — twenty-four pages — yielding a compact book typically about five by seven and a half inches in height. The term also refers to an actual book produced in this format. Duodecimo belongs to a family of format names derived from Latin ordinals, alongside folio, quarto, octavo, and sextodecimo, each named for the number of folds or leaves per sheet. Popular from the seventeenth through the nineteenth centuries, duodecimos were prized for their portability, making them the favoured size for novels, prayer books, and poetry collections intended to be carried and read anywhere. Today the word survives chiefly in the vocabulary of librarians, bibliographers, collectors, and dealers in rare books, usually abbreviated 12mo. It lends writing about books a precise, scholarly air.
nounA book size resulting from folding a printed sheet into twelve leaves (twenty-four pages), typically measuring about five by seven and a half inches; also, a book of this size.
- A page size obtained by folding a printer's sheet into twelve leaves, producing a book roughly five by seven and a half inches tall.
- A book printed in this format.
"Her first novel was issued as a slim duodecimo that fit easily into a coat pocket."
"The library's rare books room holds several seventeenth-century sermons bound in duodecimo."
"Collectors prize the duodecimo edition for its compact, portable format."
The regular plural duodecimos is standard when referring to individual books; when describing format rather than count, the singular is used in phrases such as 'in duodecimo'.
"The shelf held a row of leather-bound duodecimos from the early nineteenth century."
In the 1700s and 1800s, the duodecimo was the paperback of its day — small enough to slip into a pocket, which is why so many beloved novels of that era were 'little twelvemos.'
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of duodecimo
The word duodecimo comes from Latin duodecimus, meaning 'twelfth', the ordinal form of duodecim ('twelve'). It entered English in the late seventeenth century as part of the printing trade's system of naming book formats after the number of leaves produced from a folded sheet. The same Latin root gives English words such as duodecimal (relating to twelfths) and duodenal (from the intestine 'twelve fingers long'), and it lies behind the Germanic-derived dozen.
Related word forms
How duodecimo is actually used
Duodecimo is technical vocabulary from descriptive bibliography and bookselling, largely confined to cataloguing, antiquarian book trade, and literary scholarship. It is often abbreviated as 12mo and used adjectivally in phrases such as 'a duodecimo volume' or 'in duodecimo'. The word carries a scholarly, slightly archaic flavour in general prose.
Easily confused with duodecimo
An octavo is a larger book format made by folding a sheet into eight leaves (sixteen pages), whereas a duodecimo is folded into twelve leaves (twenty-four pages) and is correspondingly smaller.