A paperback (also known as softback or softcover) is a type of book characterized by a thick paper or paperboard cover, and often held together with glue rather than stitches or staples. In contrast, hardcover or hardback books are bound with cardboard covered with cloth; although more expensive, hardbacks are more durable. Inexpensive books bound in paper have existed since at least the 19th century in such forms as pamphlets, yellowbacks, dime novels, and airport novels. Modern paperbacks can be differentiated by size. In the US there are "mass-market paperbacks" and larger "trade paperbacks." In the UK, there are A-format, B-format and the largest C-format sizes.