\Quar"to\, 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.

\Quar"to\, 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.