Pronunciation: (kwôr'tō), [key]
—n., pl. -tos,
1. a book size of about 9½ × 12 in. (24 × 30 cm), determined by folding printed sheets twice to form four leaves or eight pages. Symbol: 4to, 4°
2. a book of this size.
bound in quarto.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.