Folio, a term from bibliography, refers to the format of a book. A folio is the largest format; a sheet is folded over only once, so each sheet consists of two leaves (or four pages). A folio might be as much as two feet from the head of the page to the foot.

Traditionally, the most serious and dignified materials are published in folios (the Bible, dictionaries, scholarly works), while the smaller formats are reserved for more popular and ephemeral works.

From the Guide to Literary Terms by Jack Lynch.
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Note: This guide is still in the early stages of development.
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