Du`o*dec"i*mo (?), 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.

Du*o*dec"i*mo, 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°.