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.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)