The size of a book whose pages are made by folding a sheet of paper twice to form four leaves.
book size made by folding sheet into four leaves: foolscap quarto, 81/2 × 63/4 in; crown quarto, 10 × 71/2 in; demy quarto, 111/4 × 83/4 in; royal quarto, 121/2 x 10 in; imperial quarto, 15 x 11 in (abbr. 4to).