Folios in the context of "Leaf (books)"

The term "folio" (from Latin folium 'leaf') has three interconnected but distinct meanings in the world of books and printing: first, it is a term for a common method of arranging sheets of paper into book form, folding the sheet only once, and a term for a book made in this way; second, it is a general term for a sheet, leaf or page in (especially) manuscripts and old books; and third, it is an approximate term for the size of a book, and for a book of this size.

First, a folio (abbreviated fo or 2) is a book or pamphlet made up of one or more full sheets of paper, on each of which four pages of text are printed, two on each side; each sheet is then folded once to produce two leaves. Each leaf of a folio book thus is one half the size of the original sheet. Ordinarily, additional printed folio sheets would be inserted inside one another to form a group or "gathering" of leaves prior to binding the book.