duodecimoEtymologyLatin ablative of duodecimus (“twelfth”) (from duodecim 'twelve', from duo 'two' + decem 'ten')nounA size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.A sheet or page of that size.A book having pages of that size.“In recent years, I have taken to reading while walking. I believe the Romantics were guilty of such behavior; a duodecimo was small enough to fit into a jacket pocket.”