ˈkwȯr]d.(ˌ)ō, -ȯ(ə)], ](ˌ)tō\ noun
( -s )
Etymology: Latin, abl. of quartus fourth
1. : the size of a piece of paper cut four from a sheet ; also : paper or a page of this size — abbr. 4to ; symbol 4° ; see book tables
2. : a book printed on quarto pages
the volume is a quarto , bound on the narrow side — Walter Millis