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The Library of Congress, the largest library in the United States, contains about eighteen million books. Harvard University has the largest academic library with more than nine million volumes. The largest public library is the Chicago Public Library with nearly six million volumes, followed by the New York Public Library with 5. 5 million. Consequently, finding a book in any of these libraries must be like finding a needle in a haystack!
   Librarians use a special system to make the job of finding books easier. To keep the books in order, librarians give a number to each book in the library, and books are arranged on the shelves according to these numbers. In order to find the number of a book, you must look in the card catalogue. The card catalogue is a special file of cards with information about the books in the library. Each book has three cards in the card catalogue: a title card, an author card, and a subject card. These cards are arranged in the card catalogue in alphabetical order. If you know the title of the book you are looking for, look for the title card. If you only know the author look for the author card. If you are looking for a book about a particular subject, look for the subject card. When you find the card of the book you want, copy the number of the book. Then you can find the book easily.