The term folio derives from the Latin word “folium” (leaf) and refers to a traditional book format in which the size of the page is determined by the fact that the sheets used in the production of the book are folded only once. An unfolded sheet is called an atlas format, double folio or large folio.
However, because the size of the initial sheet and thus the final size of the book can vary, librarians have established their own guidelines for categorizing book formats. In Germany, modern library categorization follows the guidelines of the Deutsche Bibliothek in Frankfurt am Main, which date back to the Prussian Instructions (PI). Although this falls back on the old book format designations, it defines them exclusively by the height of the spine. Today, books with a spine height of 40-45 cm are classified as folio format. Books in folio format are marked with the abbreviation fol. or indicated as 2° (for 2 sheets).
World-famous examples of books in folio format are the Gutenberg Bible and Shakespeare’s First Folio of 1623, which was printed in 1,000 copies, bound in calfskin and sold for the price of 1 pound. Of course, only very wealthy people could afford this book.
In modern book printing, the folio format is mainly used for elaborate illustrated books or faithful reprints (facsimiles) of precious books from past times. The folio format is also popular for documenting anniversaries: many companies, corporations or institutes prefer to document their (success) history in large format. These volumes are then often published as hand-picked editions.
Since the classic folio format does not conform to any DIN standard, but lies between the formats DIN A2 and DIN A1, it also places special demands on scanning systems during digitization.