1911 Encyclopædia Britannica/Quarto

QUARTO, a shortened form of Lat. in quarto, “in a fourth,” i.e. of a sheet of paper, applied to a size of paper, and to a size of a printed volume. Paper is in quarto when a whole single sheet is folded twice so as to form four leaves; a book is technically termed of “quarto” size when made up of sheets folded twice.