A paperback, also known as a softcover or softback, 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, plastic or leather. The pages on the inside are made of paper. 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 U.S., there are "mass-market paperbacks" and larger, more durable "trade paperbacks". In the U.K., there are A-format, B-format, and the largest C-format sizes. Paperback editions of books are issued when a publisher decides to release a book in a low-cost format. Cheaper, lower quality paper; glued (rather than stapled or sewn) bindings; and the lack of a hard cover may contribute to the lower cost of paperbacks. Paperbacks can be the preferred medium when a book is not expected to be a major seller or where the publisher wishes to release a book without putting forth a large investment. Examples include many novels, and newer editions or reprintings of older books. Since paperbacks tend to have a smaller profit margin, many publishers try to balance the profit to be made by selling fewer hardcovers against the potential profit to be made by selling more paperbacks with a smaller profit per unit. First editions of many modern books, especially genre fiction, are issued in paperback. Best-selling books, on the other hand, may maintain sales in hardcover for an extended period to reap the greater profits that the hardcovers provide.