Folios are very tall books—15 inches or more.  Elephant folios are up to 23 inches in height.  Atlas folios, up to 25 inches.  There are even double elephant folios, which can be up to 50 inches tall!

By the mid-19th century, folios were made with state-of-the-art printing technology.   Larger sheets of paper and new roller presses allowed the creation of these oversized books.

Typically these folios were scientific and scholarly works intended for a very specialized audience, both because of the subject matter,  and also the cost.  James Audubon’s Birds of America is a classic example.  Imagine these big books spread out on tables in wood-paneled private libraries.