The short answer is: Using a classification system groups all of the information in your library together by subject so people can find things easier. It gives a book an address so it always returns to the same place on the shelf.
Let me digress a minute here to define the difference between cataloging and classification. Cataloging is a verbal description of the book: author, title, publisher, date, number of pages, subject headings. Cataloging creates the index. You can catalog your entire library without ever classifying it.
Classification is how you place the material on the shelf -- how you sort the information to make sense out of it. Classification is the address -- it's the block number and house number so you can find something.
The Dewey system is one way to organize knowledge. In the United States, 95% of all public and school libraries, 25% of college and university libraries and 20% of special libraries use Dewey. More than 200,000 libraries worldwide in 135 countries count on Dewey Classification to keep their collections in order. Because it is so widely used, once people understand it, they can move from the smallest library to the largest and know how to find things.
Melvil Dewey, who developed the Dewey Classification in 1876, set it up to handle ALL kinds of knowledge. His plan was to have a logical method to organize information, even information that did not exist in 1876, such as computers and space flight. He did this by dividing all knowledge into ten broad categories that were each given a number code. These categories are:
000 Computer science, Information and General works
100 Philosophy and Psychology
200 Religions
300 Social Sciences
400 Languages
500 Science and Mathematics
600 Technology and Applied science
700 Arts and Recreation
800 Literature
900 History, Geography, and Biography
Fiction falls into this classification scheme as 800: Literature. Most libraries find that fiction grows too fast to keep it in the 800s, however. If fiction is kept it in the Dewey numbers, the library would constantly need to shift shelves to make room for the new books. Instead, librarians pull fiction out of the Dewey scheme and move it to another spot in the building. If there is demand, they might break the fiction collection into subgroups according to genre: mysteries, westerns, science fiction, romance, etc. The thing to remember it there is still a place for fiction in the Dewey Classification, but the individual library has chosen not to use that number. The numbers for fiction are usually saved for literary criticism and works about authors rather than their works.
With that said, how does the classification system work? Knowledge is organized from general to specific. A broad category might be 600: Applied Science. Every digit after the 6 moves to a more specific level of information: 610 is medicine; 620 is engineering; 630 is agriculture, etc.
The next digit is a more specific 634 = Orchards, fruits, forestry; 635 = Garden crops ( Horticulture); 636 = Animal husbandry. Each number is more specific than the last, fine tuning the information grouped there. They can get pretty fine, too! The longest number I've seen had 21 places after the decimal point.
There are also some code numbers called Standard Subdivisions that almost always mean the same thing: 03 = dictionaries; 092 = biography; 973 = United States; 064 = professional sports, etc.
When you see long call numbers on books, they are often created by grouping some of these standard subdivisions after a “regular” number.
796.33 (Sports played with balls)
796.332 (Football)
796.332064 (Professional sports)
796.332064973 (United States)
796.332064973092 (Biography)
S0 796.3320647973092 is the grouping for all biographies of American professional football players. If you use this number for all of these books, your patrons will find all of those books together on the shelf.
Only big research libraries use the really long numbers. Most other libraries have a policy to only use 3 digits past the decimal place or to stop at the first reasonable break. In this case, that could be 796.332 (3 digits past the decimal or first significant break) or they might drop some of the intermediate information and use 796.332 (football) 092 (biography) to get 796.332092. (Ok, dyed-in-the-wool catalogers may quake at leaving something out, but this is practical cataloging. :) )
So you have assigned a Dewey number to an item. Is that all there is? No. Not usually. The Dewey number is the block number that gets someone in the right area to find a topic, but you need more to get them to a specific book. Charles Ammi Cutter, developed a different classifcation system about the same time Melvil Dewey developed his system. Many libraries use a variation of Cutter's system called the Cutter-Sanborn Table to create a author code to go with the Dewey Number. This is called a Cutter Number.
Assume you have a book called World War II by Simon Adams. The Dewey number is 940.53 (World War II). To create a Cutter number, you would start with a capital A (Author's last name) add the Cutter-Sanborn code (217) and finish with a small case w for the title. The Cutter number would be A217w. This would be written on the second line after the Dewey number and make the complete call number:
940.53
A217w
Cutter numbers are very specific and librarians and shelvers love them. Patrons don't understand them. You will find patrons in the middle of the 200s (Religion) looking for A217w, the World War II book. Because of that, many libraries have quit using "real" Cutter numbers but still call the information they put in the second line of the call number a "Cutter." Most frequently, the second line information is the author's last name or some part of the last name. (I recommend using the entire last name rather than X number of letters. It's more specific.)
940.53
Adams
There you have it: why we use Dewey numbers. They are the address of the book to help patrons locate them on the shelf.
