Once he arranged all the books, Mr. Dewey and his friend, Thomas Edison, developed a way to find the information without much help from the librarian. They used an old idea from France to make a catalog using index cards. Each book had an index-sized card on which the title, author, publication information, and possibly a summary of the book.
Then Mr. Dewey and Mr. Edison wrote several copies of the same card with different headings: one for the title of the book, one for the author of the book (last name first, please!) and one or more for the subjects that the book was about.
Here is a stack of four cards ready to be filed:author card, title card, and two subject cards:
He then arranged the cards in alphabetical order according to the heading on the card. They used a large cabinet with many drawers and called it the card catalog. The cards allowed the librarians to add and take away the cards as they acquired (added) and discarded (removed) books from the library.
cc: "A card catalog in the University Library of Graz" by Marcus Gossler, 2005.
We still use these headings today in the OPAC -- the electronic (computer-based) version of the card catalog. We can think of the old card catalog as a paper version of a database.