Traditional sizes
Book sizes describe how the printed sheet was folded: a folio (folded once, largest), a quarto (4to) (folded twice), an octavo (8vo) (folded three times, the common modern hardcover size), and a duodecimo (12mo) (smaller still). The terms survive in catalogue descriptions even though modern books are machine-made.
Modern formats and first editions
Today the formats that matter for identification are the trade hardcover, trade paperback, and mass-market paperback. For many books — especially science fiction and crime of the mid-20th century — the first appearance was a paperback original, which makes that mass-market the true first edition. Don’t assume the hardcover is the first.
Frequently asked questions
What size is an octavo?
An octavo (8vo) is the common modern hardcover size, made by folding the sheet three times. Quarto (4to) is larger; duodecimo (12mo) is smaller.
Can a paperback be a first edition?
Yes. When a book first appeared as a paperback original, that mass-market paperback is the true first edition — common for mid-century science fiction and crime.