A hardcover book also known as hardback is a book bound with unbending protective covers. They may have flexible, sewn spines which allow the book to lie flat on a surface when opened, or have glued spines. Following the ISBN sequence numbers, these types of books are abbreviated by Hbk. Covers are usually made of cardboard covered with cloth, heavy paper, buckram, or occasionally leather). Hardcover books are often printed on acid-free paper, and are more durable than paperbacks. Hardcover books are more costly to produce and are more expensive. They frequently come with artistic dust jackets but a jacketless alternative is the dust jacket in favor of printing the cover design directly onto the board binding.