fo

In printing, fo stands for folio, a page size created by folding a single sheet of paper in half. The resulting leaf measures roughly 10 inches by 15 inches, though exact dimensions vary by the parent sheet size and the paper stock used. A folio book opens to show two pages side by side, each one half the area of the original unfolded sheet. This fold produces four page surfaces: two on the front and two on the back of the folded leaf.

The term comes from the Latin word for leaf, and printers still use it to describe both the physical format and the signature or gathering that results from the fold. A single fo signature contains four pages numbered consecutively. When multiple folios are nested inside one another and sewn at the spine, they form the text block of a book. Knowing a book's format, fo tells a compositor or binder immediately how the original sheets were divided and folded during production.

Folios sit at the middle of the standard size hierarchy. Quarto, or 4to, results from folding twice and yields four leaves per signature; fo comes from a single fold; and folio in the broadest sense describes any book made from leaves folded from larger printing sheets. A broadside, by contrast, is one unfolded sheet. Crown folio and demy folio refer to fo made from different parent sheet sizes, producing slightly different finished dimensions.

In historical printing, folio became synonymous with serious, substantial books, especially works of scholarship, theology, or law. The Shakespeare First Folio of 1623 is the archetypal example. Today, fo remains in use in rare book cataloging and in hand bookbinding, where understanding the original gathering structure matters for conservation and reproduction. Digital designers and modern publishers rarely invoke the term, but anyone working with period books or hand-sewn bindings will encounter it constantly.