du·o·dec·i·mo 
(d′ə-dĕsə-mō′, dy′-)
(d′ə-dĕsə-mō′, dy′-)n. pl. du·o·dec·i·mos
1. The size (5 by 7 3/4 inches) of book pages formed by folding single sheets from a printing press into 12 leaves each.
2. A book composed of pages of this size. In both senses also called twelvemo.
[Latin (in) duodecimō, (in) a twelfth, ablative of duodecimus, twelfth; see DUODECIMAL.]