Definition of term:: Duodecimo

  • Often abbreviated as 12mo. This doesn't mean "a dozen more," but in the book world describes a small book (a little smaller than a mass market paperback), whose printed paper sheets were folded in such a fashion as to make twelve leaves (24 little pages) from one large sheet. And yes, it is sometimes pronounced "twelve-mo."
French: in 12
German: Duodez
Source: What bookdealers really mean. A dictionary by Tom Congalton and Dan Gregory (Between the Covers), with additions from other sources