Folio, << FOH lee oh >>, is the name printers and publishers use for a sheet of paper folded once, making four pages, front and back. The word folio may also mean the page number of a book. Even-numbered pages, or folios, are always on the left. Odd-numbered folios are on the right side of the bound book or volume.
A quarto is a sheet folded twice, making four leaves or eight pages. An octavo is a sheet folded into eight leaves or 16 pages. The octavo format, or shape of the book, is the one in most common use. Folio format or octavo format formerly was used to relate the size of a book. But size today depends on the dimensions of the sheet before it was folded.
