The Dewey Decimal system is a classification scheme for organizing the library's books on the shelves. It was conceived in 1873 by Melvil Dewey (1851-1931), while he was assistant librarian at Amherst College, and first published in 1876. Today it is used in more than 135 countries, and most public and school libraries in the United States. .
The system divides knowledge into ten main classes which are further subdivided decimally to form a total of 1000 categories numbered 000-999. Books are assigned 'call numbers' according to their relationship to one another. In other words, you will find that books on the same subject will be filed close to each other on the shelf. If, for example, you are looking for books on North American birds, you would find them all grouped together. Prior to the Dewey Decimal system, books were assigned numbers according to their physical location on a shelf.
Every book in the library is assigned a unique number which is used to locate the book. You can think of it as the book's address. Call numbers are assigned according to the Dewey Decimal Classification schedule.