The document introduces the Dewey Decimal System, which was invented by Melvil Dewey to organize the subjects in a library. It explains that nonfiction call numbers contain the Dewey numbers corresponding to the subject, followed by the first three letters of the author's last name. It provides examples of the 10 main subject categories in the Dewey system and the numbers assigned to each one, such as 100s for Philosophy, 200s for Religion, and 900s for History/Geography.