Paperback, softback or softcover describe and refer to a book by the nature of its binding. The covers of such books are usually made of paper or paperboard, and are usually held together with glue rather than stitches or staples. Inexpensive books bound in paper have existed since at least the 19th century and exist in a number of formats that have specific names, such as pamphlets, yellowbacks, dime novels and airport novels. Most paperbacks are either "mass-market paperbacks" or "trade paperbacks". In contrast, books with hardcover or hardback are bound with cardboard covered with cloth; although more expensive, hardbacks are more durable.