a. [ L. in duodecimo in twelfth, fr. duodecimus twelfth, fr. duodecim twelve. See Dozen. ] Having twelve leaves to a sheet; as, a duodecimo from, book, leaf, size, etc. [ 1913 Webster ]

n.; pl. Duodecimos A book consisting of sheets each of which is folded into twelve leaves; hence, indicating, more or less definitely, a size of a book; -- usually written 12mo or 12°. [ 1913 Webster ]