A bookcase, or bookshelf, is a piece of furniture, almost always with horizontal shelves, used to store books. It may be fitted with glass doors. A bookcase consists of a unit including two or more shelves which may not all be used to contain books or other printed materials. Shelves may be fixed or adjustable to different positions in the case. In rooms entirely devoted to the storage of books they may be permanently fixed to the walls and/or floor.