“What is the difference between a folio and a quarto? An octavo and a duodecimo? All of these terms refer to book sizes. In the first centuries of printing, book pages were of a standard size—13½ inches by 17 inches. These ‘foolscap’ sheets, when folded one or more times, produced a ‘signature,’ a section that was bound with other signatures to produce the book. A folio was a signature of two leaves, a quarto four leaves, an octavo eight leaves, and a duodecimo twelve leaves.”
Excerpted from: Corey, Melinda, and George Ochoa. Literature: The New York Public Library Book of Answers. New York: Simon and Schuster, 1993.