Back in the early days of printing and paper manufacturing, there weren’t standard sizes yet. On the other hand, they did have standard names for a book’s format:
A press sheet folded in half was called a folio. You get 2 leaves or 4 pages.
If you folded the press sheet again, you get a quarto—4 leaves or 8 pages. It’s half as big as a folio.
If you folded the press sheet one more time, you get an octavo—8 leaves or 16 pages. It’s one-fourth as big as a folio. 
Of course you can keep folding, at least until you get so many pages the paper won’t bend anymore. These formats get called duodecimo, sextodecimo, octodecimo…
Back to the beginning of The Western Civ User’s Guide to Reading & Writing.
Don’t forget: I wrote another Western Civ User’s Guide! Back to the beginning of The Western Civ User’s Guide to Time & Space.
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