"It's the Dewey Decimal System."
Dewey Decimal System
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The Dewey Decimal System (or the Dewey Decimal Classification) is a system of library classification developed in 1876 by Melvin Dewey. This system organizes books on library shelves in a specific order that makes it easy to find any book and return it to its proper place. The system is used in 200,000 libraries in at least 135 countries.