In the article: In the New Game of Tag, All of Us Are It, Newsweek's Steven Levy talks about the rise of interest in tagging.
This following is not a rehash of those issues or another post arguing the pros and cons of tagging. We can save that for later. It's just a few facts about cataloging information.
Whereas the old, Dewey-style taxonomies involved graybeards figuring out in advance how things should be categorized, tagging is done on the fly, adapting to the content itself. What's more, because all this is digital, there's no limit to the number of tags people can slap on an item. In a library you can put "Frederick the Great" in the history or the biography section, but you'd need a second copy to put it in both. With digital tags you could use both, and more: military,Prussia, really great reads.
First, I think it's time to end comparing everything to the Dewey Decimal System. Many libraries (school and public) use Dewey but many others don't. Another classification scheme called Library of Congress Classification (LCC) is used in many large libraries around the world. Also, many of the people who do this type of work are not graybeards. (-: Hey, this week is National Library Week. (-:
Second, Dewey and LCC are used to determine where a book goes in the stacks. These numbers do reflect what the book is about and yes, most books have just one classification number assigned to them. However, the same book might have several subject headings assigned to it (more on that below). Btw, many people don't realize that online library catalogs (aka OPACS) allow you to browse by classification number just like you would browse the stacks looking for items.
What Levy gets wrong is that books and other objects are also assigned what Library of Congress Subject Headings. A book can have many subject headings assigned to it just like a bookmark in de.lico.us can have many tags. When you do a subject search in a library catalog you're most likely searching on these headings. Of course, cross references are also a part of these headings. For example, the book I co-authored with Chris titled, The Invisible Web is assigned the following headings from one library:
+ Online databases--Directories.
+ Database searching
+ Internet searching
Here's one more example:
Title: Eisenhower and Churchill : the partnership that saved the world
This book is assigned five subject headings:
+ Eisenhower, Dwight D. (Dwight David), 1890-1969
+ Churchill, Winston, 1871-1947
+ World War, 1939-1945--Biography
+ Prime ministers Great Britain Biography
+ Generals United States Biography
Local libraries can create their own subject headings or modify what the Library of Congress provides. You can learn more about Library of Congress Subject Headings here. If you scroll down to "The Weekly Lists" you can see lists of headings as they join the vocabulary or are removed from it.
You can see and use subject headings and other parts of a catalog entry to refine your search when you use RedLightGreen, a database of over 120 million books. You can even use RLG to find items in thousands of public and academic libraries. An overview here.
Finally, many large database providers have their own vocabularies that indexers choose subjects (often referred to as descriptors) from when indexing articles. Often, articles are assigned many descriptors. ere's a link to the vocabulary that ProQuest uses.
More about "authority control" (another issue) and a vocabulary for images here and here.