size of paper or page (more or less 6.5 to 7.5 inches high and 4.5 inches wide), 1650s, from Latin in duodecimo (folded) "in a twelfth" of a sheet, from ablative of duodecimus "twelfth," from duodecim "twelve" (see dozen). So called because made originally by folding a printer's sheet and cutting it in 12 leaves. Often abbreviated 12mo. Also "a book in which each page is the twelfth part of the printer's sheet." Related: Duodecimary.