These terms are used to describe books produced by traditional methods of paper making and printing, before the beginning of the 20th century, and they refer to the way a sheet of paper is folded (also known as the format) and cut to become part of a printed book.

A sheet folded once gives two leaves, that is four pages, each measuring half the surface of the sheet. Likewise, a sheet folded twice results in four leaves and a sheet folded three times results in eight leaves. A folio (from Latin folium, which means ‘leaf’) is a book made of sheets folded one single time. A book consisting of sheets folded twice is called a quarto (from Latin quartus ‘a fourth’), because its leaves measure a quarter of a sheet. Similarly an octavo is a book consisting of leaves that measure one eighth of a sheet.

The actual measurements of a folio, quarto, etc., depend, of course, on the size of its original sheets.