The Dewey Decimal System (DDS) is a classification system used to organize and catalog books and other materials in libraries. There are other such systems, like the Library of Congress Classification system. But the DDS and its modern offshoot the Universal Decimal Classification, are by far the most commonly used systems in libraries around the world.
The DDS system doesn't organize books so much as create a framework by which all possible human knowledge can be classified into branches. Indeed it's a testament to the system's extensibility that it was invented before electronics and computers, before aviation, before atomic theory and dozens of other basic elements of modern life. Despite the huge explosion in knowledge during the twentieth century, the DDS still works just fine.
The DDS breaks subjects down into ten classes (thus "decimal"). Each class is divided into ten divisions, and each division into ten sections. Each class, division and section is represented by a number, and these are the numbers you see on the spines of library books. Determining the exact subject matter of a new book will determine what number it should receive. And since libraries are organized by the system, knowing a book's number, or even how its subject would be classified, helps locate the book on the shelves.
The DDS is named after its inventor, Melvil Dewey (1851-1931). Dewey was devoted to books, and obsessed with efficiency. He invented hanging files, and even went so far as to change his name to Melvil from the original Melville because he considered the last two letters unnecessary. He developed the original version of the DDS while a student and assistant librarian at Amherst College in Massachusetts. As always, Dewey wanted to make the library more efficient. There was little standardization in libraries of the time, and most systems that existed were arranged around physical locations. They told you what shelf a book would be found on. The number of published books was skyrocketing at the time, and these systems were quickly overwhelmed. Dewey's system improved on them by referring not to a physical shelf but a logical branch of knowledge. This made the system more efficient and especially made it better at absorbing and rearranging all the new books flowing into libraries.
Dewey's system quickly grew in popularity with libraries. Dewey patented the system, and it remains privately owned though it has changed ownership several times. Since 1988, it has been held by the Online Computer Library Center, a non-profit cooperative that produces library systems and catalogs.
The DDS has been translated into more than 30 languages, an effort that is continuing today. The system itself has been revised repeatedly over the years as libraries gained experience with it. Dewey himself presided over 13 revisions until his death in 1931. The most recent revision, the twenty second, was released in 2004.
At its most basic level, the DDS divides all books into ten basic Classes. Thus, if you can decide which class a given book falls into, you already know which range of numbers it will fall within and can start tracking it down based solely on that. The ten basic classes and their DDS numbers are:
000--Computer science, information, general works
100--Philosophy and psychology
600--Technology and applied science
700--Arts and recreation
900--History and geography and biography
You can break subjects down farther within these ranges, replacing zeroes with numbers from the system. For example, within 500 (science), 510 is mathematics and 516 is geometry.
The DDS made it possible for libraries to function smoothly and effectively at a time when many more books were being published, and philanthropists like Andrew Carnegie sought to build libraries across the US to help educate a booming population. Thus the DDS is in large part the reason why your community has a public library today. Fully 95 percent of American public school libraries use the system, though colleges are more likely to use the Library of Congress system. The DDS is so important to the modern library that Dewey is considered to be the father of the field of library science.
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