The word "folio" comes from the Latin word "folium," which means "leaf." In the world of books and printing, "folio" has three related but different meanings. First, it refers to a way of making books by folding a single sheet of paper once, creating a book made this way. Second, it is a general term for a sheet, leaf, or page in manuscripts and older books. Third, it is a term used to describe the size of a book, usually about 15 inches (38 cm) tall, though it does not always match the actual size of the book.

A folio (shortened as "fo" or "2") is a book or pamphlet made from one or more full sheets of paper. Each sheet has four pages printed on it—two on each side. After folding the sheet once, it creates two leaves. Each leaf is half the size of the original sheet. Additional sheets are often placed inside one another to form a group, called a "gathering," before the book is bound.

In some books and manuscripts, "folio" is used to describe pages. Instead of using numbers, the terms "recto" and "verso" are used to mean the front and back sides of a leaf. This is written as "f26r," which means the front side of the 26th leaf. For books written in scripts read from left to right, like Latin or Greek, the recto side is on the right. For books written in scripts read from right to left, like Hebrew or Arabic, the recto side is on the left.

The word "folio" is also used to describe a book's size. Books called "folio" are usually about 15 inches (38 cm) tall. However, this term does not always match the actual printing format of the book, and some modern books may not have a known format. Other common book sizes include "quarto" and "octavo," which are made by folding a sheet two or three times, respectively.

Examples of famous folios include the Gutenberg Bible, printed around 1455, and the First Folio, a collection of Shakespeare's plays printed in 1623. However, these books are not the same size as the typical folio.

Format

A folio is a type of book or pamphlet made from one or more full sheets of paper. Each sheet has four pages of text printed on it—two pages on each side. After printing, each sheet is folded once to create two leaves. Each leaf of a folio book is half the size of the original sheet. This is different from a quarto, which folds each sheet twice, and an octavo, which folds each sheet three times. Unlike a folio, books made as quarto or octavo often need their pages cut open after being bound. This cutting was sometimes done by the reader using a tool called a paper-knife, especially in older books.

There are different ways to make folios. For example, a book printed as a folio but bound in groups of eight leaves is called a "folio in 8s" by experts who study books.

The Gutenberg Bible, printed around 1455, was made as a folio. Each sheet of paper had four pages of text, which were folded once. The pages were about 12 inches by 17.5 inches (307 mm by 445 mm), a size called "double folio." Multiple folded pairs of leaves were placed inside each other to form sections, which were then sewn together to create the final book. Shakespeare's First Folio, printed as a folio, has pages that are 12.5 inches tall (320 mm), which is smaller than the Gutenberg Bible's folio size.

Folios were a common format for books printed during the incunabula period, which refers to books made before 1501. However, the earliest surviving printed book, which only exists as a fragment of a leaf, was a quarto. The British Library's Incunabula Short Title Catalogue lists about 28,100 different editions of books, pamphlets, and broadsides printed before 1501. Of these, about 8,600 are folios, which makes up just over 30 percent of all works in the catalog.

Page numbering

When talking about manuscripts, a folio is a single leaf that has two pages: the recto, which is the page a reader sees first, and the verso, which is the second page. In Western books, which are read by turning pages from right to left, the first page seen when a book is opened is called "folio 1 recto" and is often written as "f1 r." When this page is turned, the next page on the left is "f1 v." and the page on the right is "f2 r." These two pages together form an "opening," which is the pair of pages visible when a book is open. For books in Arabic, Hebrew, Japanese, and other languages, the book starts from the back in Western terms, with the open edges of the pages on the reader's left. In these cases, the numbering follows the order in which the reader sees the pages.

When discussing manuscripts with two columns of text, letters like a/b/c/d may be used to describe the left and right columns of recto and verso pages. For example, "f. 150a" and "f. 150b" refer to the left and right columns on the recto page, while "f. 150c" and "f. 150d" refer to the left and right columns on the verso page. In manuscripts with three columns, the notation may include the folio number, recto or verso, and a column letter. For example, "f. 3 v. col. c" refers to the third column on the verso side of the third folio.

Size

The actual size of a folio book depends on the size of the paper used to print it. In the past, paper sizes were not the same, so the meaning of "folio" is only approximate. Historically, printers used names to describe paper sizes, with approximate maximum page heights:

• Double Elephant Folio (50 inches, 127 cm)
• Atlas Folio (25 inches, ≈ 63 cm)
• Elephant Folio (23 inches, ≈ 58 cm)
• Royal Folio (20 inches, ≈ 51 cm)
• Medium Folio (18 inches, ≈ 46 cm)
• Crown Folio (15 inches, ≈ 38 cm) (the most common).

Starting in the mid-nineteenth century, new technology allowed the creation of large paper sheets or rolls for printing books, with many pages printed at once. This made it very difficult to know the exact format (the number of pages made from each sheet used in a press). Today, the term "folio" for such books may only describe the size, meaning books that are about 15 inches (38 cm) tall.

In modern times, the term "folio" often refers to "foolscap folio," a type of paper measuring 8.5×13.5 inches (216×343 mm), which is slightly larger (by 18.7%) than A4 paper.

Shakespeare folios

From the earliest times of printing, large, expensive books called folios were often used for important works. In the 1600s, plays from the English Renaissance theatre were printed in collections called folios. For example, 36 of Shakespeare's plays were included in the First Folio, a collection published in 1623. More folio editions followed, such as the Second Folio and others. Other writers of this time also published their plays in folio collections, like Ben Jonson's works, which were gathered in 1616.