A folio usually indicates a large book size of 15" in height or larger when used in the context of a book description.

Further, folio sizes are often sub-divided and described as being one of:

  • Crown folio (15" - 18")
  • Medium folio (18" - 20")
  • Royal folio (20" - 23")
  • Elephant folio (23" - 25")
  • Atlas folio (25" - 50")
  • Double elephant folio (50" and up)
However, folio also has a distinct meaning when describing a type of binding: a folio is a method of binding sheets of paper into leaves, whereby a single printed sheet of paper is folded once. Several folios are usually laid inside of one another, creating a gathering which is in turn stitched into a book.

Folio is often abbreviated fo or 2░.

This 2 volume survey of London from 1720 is a good example of a folio:

example of a folio