/kwawr"toh/ , n. , pl. quartos , adj.
n.
1. a book size of about 9 1 / 2 × 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.
adj.
3. bound in quarto.
[ 1580-90; short for NL in quarto in fourth ( quarto, abl. sing. of quartus FOURTH) ]