A bookcase, or bookshelf, is a piece of furniture, almost always with horizontal shelves, used to store books. Bookcases are used in private homes, public and university libraries, offices and bookstores. A bookcase 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. Bookcases range from small, low models the height of a table to high models reaching up to ceiling height. 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. Bookcases may have doors that can be closed to protect the books from air pollution, and bookshelves are open-fronted. These doors are almost always glazed, so as to allow the spines of the books to be read. Especially valuable books may be kept in locked cases with wooden or glazed doors. A bookshelf normally stands on some other piece of furniture such as a desk or chest. Larger books are more likely to be kept in horizontal piles and very large books flat on wide shelves.