Libraries use a system called the dewey decimal system to put books into a specific order and label them so that they are easy to find when someone is looking for a specific book. This system was first used by Melvil Dewey in 1876. It has been revised and expanded as the years have gone by.
As an example of how the system works a book that is labeled R REF 95.1 351.4 FEL LAM would be kept in the reference section. The system is able to organize all the books into an orderly manner even in a large library containing thousands of different books.