A hardcover, hardback, or hardbound book is one bound with rigid protective covers (typically of cardboard covered with cloth, heavy paper buckram, or occasionally leather). It may have a flexible, sewn spine which allows the book it lie flat on a surface when opened. Following the ISBN sequence numbers, books of this type are identified by the abbreviation HBK. Hardcover books are often printed on acid-free paper, and are much more durable than paperbacks.