In typography, a page number, commonly placed outside the running head at the top of the page. Folios are also commonly set flush left on verso pages and flush right on recto pages. They can also be centered at the top of the page. A folio that appears at the bottom of a page is called a drop folio. A folio counted in page numbering but which is not actually printed is called a blind folio. A folio that is printed is also called an expressed folio.
In printing, a folio refers to any printed sheet that is folded once to produce four pages. See also quarto, octavo, sixteenmo, and thirty-twomo.