Historically, the term has been used to refer to any encyclopedic reference book ( that is, one comprehensive in scope ), which was organized alphabetically, as with the familiar dictionary . ( The term " dictionary " preceded " encyclopedia " in common usage by about two centuries . ) To convey their alphabetic method of organization and to contrast that method with other systems for classifying knowledge, many early encyclopedias were titled or sub-titled " a dictionary of arts and sciences " or something similar.
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