Today the names of book sizes that publishers use are a continuation of an old system. The size of the page/book is considered based on a large sheet of paper called the broadside. If you fold a broadside once, you get a folio. If you fold the folio once, you get a quarto. If you fold the quarto once, you get an octavo. If you fold an octavo once, you get a duodecimo.
This is similar to the modern system of A3, A4, and A5 size paper.
Famous folios include Shakespeare’s First Folio collection edition of plays in 1623. Printed in 1455, the Gutenberg Bible was also printed as a folio. However, the two folios were not the same actual size. The reason is that this system does not specify the size of the original sheet. So prefixes were used, such as royal (meaning large), medium (medium), and crown (meaning small) to label the general size of the paper. For example, a royal octavo meant the size was one-eighth of a royal sheet; a medium octavo meant the size was one-eighth of a medium sheet; a crown octavo meant the size was one-eighth the size of a crown sheet.
It is important to understand the history of book sizes to understand standard book sizes. These terms are still in use today. Modern books are commonly produced in folio, quarto, and octavo sizes. However, some books are made that are larger and smaller than these sizes as well. The most common book sizes are octavo and quarto sizes.
