A hardcover, hard cover, or hardback (also known as hardbound, and sometimes as casebound[1]) book is one bound with rigid protective covers (typically of binder's board or heavy paperboard covered with buckram or other cloth, heavy paper, or occasionally leather).[1] It has a flexible, sewn spine which allows the book to lie flat on a surface when opened.[1] Modern hardcovers may have the pages glued onto the spine in much the same way as paperbacks.[1] Following the ISBN sequence numbers, books of this type may be identified by the abbreviation Hbk.