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.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.