An encyclopedia (less frequently spelled encyclopaedia) is most often a compendium of writings (and related images, graphics, maps, auxillary materials) collected and stored as a book, a collection of books, or electronic media that contains systematically organized information relating to either particular fields or significant portions of human knowledge. An encyclopedia is generally used for reference purposes. Famous encyclopedias include Encyclopaedia Britannica Eleventh Edition (1911), Encyclopaedia Britannica, Encarta, World Book, Encyclopedia Americana, Wikipedia and Citizendium. Many specialized encyclopedias have appeared, such as the Catholic Encyclopedia and Jewish Encyclopedia. Since the 1990s electronic publication has increasingly replaced print. Indeed there has never been a print version of Encarta, Wikipedia or Citizendium.