The Dewey Decimal System is the most common method of classification for books in a library. Each digit in a book's classification relates to the book's content, in varying degrees of specificity.
The hundreds digit of a classification describes the book's content in the most general way possible. For example, if the hundreds digit is a one, the book discusses philosophy and psychology. If the hundreds digit is a five, the book is about science. If the hundreds digit is a nine, the book discusses history and geography.
The tens digit covers slightly more specific divisions. For example, if a classification begins with 18, the book specifically deals with ancient, medieval or Eastern philosophy. However, if it begins with 19, the book focuses on modern Western philosophy. Similarly, a classification beginning in 54 has to do with chemistry, while one beginning in 59 focuses more on zoology.
The ones digit further pinpoints the book's content. One example is the difference between 197 and 198; 197 applies to books discussing Soviet philosophy, while 198 applies to books about Scandinavian philosophy. Other examples are 598, which applies to books about birds, and 599, which applies to books discussing mammals.
If a library requires even more specific classifications, the Dewey decimal system has a provision to use digits beyond the decimal point. For example, 516 on its own applies to any book about geometry, but 516.3 is only applicable to books about analytic geometry.