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.
Folio is also used as a general term for books that are about Template:Convert/in tall, and as such does not necessarily indicate the actual printing format of the books, which may even be unknown as is the case for many modern books. Other common book formats are quarto and octavo.
A folio (from Latin foliō, abl. of folium, leaf ) is a book or pamphlet made up of one or more full sheets of paper, on each sheet of which four pages of text are printed, two on each side; each sheet is then folded one time to produce two leaves. Each leaf of a folio book thus is one half the size of the original sheet.
There are variations in how folios are produced. For example, bibliographers call a book printed as a folio (two leaves per full sheet), but bound in gatherings of 8 leaves each, a "folio in 8s." 
The actual size of a folio book depends on the size of the full sheet of paper on which it was printed.
The Gutenberg Bible was printed in about 1455 as a folio, in which four pages of text were printed on each sheet of paper, which were then folded once. Several such folded conjugate pairs of leaves were inserted inside one another to produce the sections or gatherings, which were then sewn together to form the final book.
Folios were a common format of books printed in the incunabula period (books printed before 1501), although the earliest printed book, surviving only as a fragment of a leaf, is a quarto. The British Library Incunabula Short Title Catalogue currently lists about 28,100 different editions of surviving books, pamphlets and broadsides (some fragmentary only) printed before 1501, of which about 8,600 are folios, representing just over 30 percent of all works in the catalogue.
Beginning in the mid-nineteenth century, technology permitted the manufacture of large sheets or rolls of paper on which books were printed, many text pages at a time. As a result, it may be impossible to determine the actual format (i.e., number of leaves formed from each sheet fed into a press). The term "folio" as applied to such books may refer simply to the size, i.e., books that are approximately 15 inches (38 cm) tall.
From the earliest days of printing, folios were often used for expensive, prestigious volumes. In the Seventeenth Century, plays of the English Renaissance theatre were printed as collected editions in folio. Thirty-six of Shakespeare's plays, for example, were included in the First Folio collected edition of 1623, which was followed by additional folio editions, referred to as the Second Folio, etc. Other playwrights in this period also published their plays in folio editions, such as Ben Jonson's collected works of 1616.
- ↑ Oxford English Dictionary, 2d ed, (1989), on line access.
- ↑ Ronald B. McKerrow, An Introduction to Bibliography for Literary Students, Oxford 1927 and later eds., p. 28.
- ↑ Search of Incunabula Short Title Catalog for imprints before 1501, sorted by date. Search done July 12, 2009.
- ↑ British Library, Incunabula Short Title Catalogue, search for imprints before 1501 and format as "fo", sorted by year. Search done July 12, 2009.
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