You may find a book description begins with some numbers and letters. Example: 4to or 8vo. These indicate the size of the book being listed. These too come out of the history of the trade and they derive from the size of the paper used to make the book.

Think of a large daily newspaper such as the New York Times. If you pull the cover page away from the rest of the paper you will have the front and last page of that day's paper. Folded so just the front page shows is the size called Folio. Now fold the paper from top to bottom. If you cut the edges, you would have FOUR pages or Quarto - often abbreviated 4to. Fold it again and you get EIGHT pages or an Octavo 8vo. Keep folding and the pages get smaller and smaller. 16mo, 32mo, 64 mo. That last is pretty small! Some people memorize these sizes; others just imagine folding a copy of a large newspaper until they get the size that would give them the cited number of pages.

You might find it useful to print out or book mark this page until you are thoroughly familiar with these terms and abbreviations.