Quarto as format
A quarto (from Latin quartō, ablative form of quartus, fourth ) is a book or pamphlet made up of one or more full sheets of paper on which 8 pages of text were printed, which were then folded two times to produce four leaves. Each leaf of a quarto book thus represents one fourth the size of the original sheet. Each group of 4 leaves (called a "gathering" or "quire") could be sewn through the central fold to attach it to the other gatherings to form a book. Sometimes, additional leaves would be inserted within another group to form, for example gatherings of 8 leaves, which similarly would be sewn through the central fold. Generally, quartos have more squarish proportions than folios or octavos.
There are variations in how quartos were produced. For example, bibliographers call a book printed as a quarto (four leaves per full sheet), but bound in gatherings of 8 leaves each, a "quarto in 8s."
The actual size of a quarto book depends on the size of the full sheet of paper on which it was printed. A demy quarto (abbreviated demy 4to) is a chiefly British term, referring to a book size of about 11.25 by 8.75 inches (286 by 222 mm), a medium quarto 9 by 11.5 inches (230 by 290 mm), a royal quarto 10 by 12.5 inches (250 by 320 mm), and a small quarto equalled a square octavo, all untrimmed.
The earliest surviving books printed by movable type by Gutenberg are quartos which were printed before the Gutenberg Bible. The earliest known one is a fragment of a medieval poem called the Sibyllenbuch, believed to have been printed by Gutenberg in 1452–53. Quartos were the most common format of books printed in the incunabula period (books printed before 1501). The British Library Incunabula Short Title Catalogue currently lists about 28,100 different editions of surviving books, pamphlets and broadsides (some fragmentary only) printed before 1501, of which about 14,360 are quartos, representing just over half of all works in the catalogue.