a. [ L. in quarto in fourth, from quartus the fourth: cf. F. (in) quarto. See Quart. ] Having four leaves to the sheet; of the form or size of a quarto. [ 1913 Webster ]

n.; pl. Quartos Originally, a book of the size of the fourth of sheet of printing paper; a size leaves; in present usage, a book of a square or nearly square form, and usually of large size. [ 1913 Webster ]