A quarto is a size of paper, and a size of book made from that paper. You take one large sheet, fold it in half, then fold it in half again. That gives you four leaves — which is why the word points at “fourth.” Print on both sides and those four leaves become eight pages. Printers, librarians, and people who study old books still say a book is “in quarto,” or call it a quarto. It is not a word for the A4 or 16K paper on your desk; it belongs to an older folding system. A folio (folded only once) is larger; an octavo (folded three times) is smaller. You will meet quarto in libraries, auctions, and notes on early printed plays — cheap single plays in Shakespeare’s time often came out as quartos, smaller and less grand than a collected folio. Everyday people just say “a small old book.” The word feels specialist, not chatty.