"It's the Dewey Decimal System."
Dewey Decimal System - Credit: Unknown
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.