A hardcover, hard cover, or hardback (also known as hardbound, and sometimes as case-bound) 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). It has a flexible, sewn spine which allows the book to lie flat on a surface when opened. Modern hardcovers may have the pages glued onto the spine in much the same way as paperbacks. Following the ISBN sequence numbers, books of this type may be identified by the abbreviation Hbk.