The term quarto is used to describe a page or book size. A printed sheet is made with four pages of text on each side, and the page is folded twice, and cut to fit inside the binding. 4 leaves, or 8 pages of text are created this way. It is one of several standard sizes of books, including folio, octavo, and duodecimo.
Traditionally, books printed in this form are large and squarish, near 10 inches wide and 12 inches tall.
The term is also associated with plays, especially Shakespeare's plays. The quarto format in The Bard's era was smaller than this style in modern books, typically under 7 inches wide and 9 inches tall. Until the eighteenth century plays generally were printed in quarto, often without the playwright's permission. Just before the turn of the (twentieth) century scholarship established that despite this many of the Shakesperean quartos were accurate.