Duodecimo (sometimes abbreviated to "12mo," and sometimes pronounced "twelve-mo"), a term from bibliography, refers to the format of a book. A duodecimo is smaller than even an octavo — about the size of a modern popular paperback. A sheet is folded over so each sheet consists of twelve leaves (or twenty-four pages). The average hardcover book today is the rough size of an octavo.

Duodecimos have traditionally been the format of choice for popular writing.

From the Guide to Literary Terms by Jack Lynch.
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