Only the SDMB would care to learn about the intricacies of library classification systems.
The Dewey Decimal System, first propounded by Melvil Dewey, the god of librarianship, served to divided up knowledge into 10 nice categories. Each categories would then be subdivided into groups of ten and further until you get some incredibly long and unwieldy numbers.
The Library of Congress System, LC, uses the letters of the alphabet along with numbers and breaks down information in to many more divisions.
The advantages of each are easy to spot. Smaller libraries can use Dewey and its relatively short numbers to divide a library’s collection into easy to use chunks. Religion? Go to the 200s. Mathematics? Go to the 510s.
As for LC, it is useful in that a big collection, like that found in a university, can be even more narrowly classified.
Need books on liquidity in economics. Go to HG 178. In Dewey, it would be, “start in the 330s and then look around a bit.”
Dewey has some problems dealing with newer types of information. Books on computer programming usually get classified in the 000s, which is where general reference books get classified. But you can also find UFOs and books on journalism there as well.
Religion may be the 200s, but Christianity takes up 200-289. All other religions go in the 290s.
The people who own the Dewey Decimal System (it’s propietary) make revisions every few years, but it’s a real pain to retrospectively change the call numbers of books, so some books “reside” under weird call numbers. The music section has been changed a lot throughout the years.
Also Dewey doesn’t handle works of fiction, so it all gets classified by the name of the author. In LC, everything gets a call number. Works of fiction are usually classified by the language and style of the work. This makes it a lot easier to find literary stuff. Dewey has a literature section (the 800s), but it’s reserved for plays, poems, essays and criticism.
There are additional classification systems. In particular medical libraries use a different one since they have so many books on medicine, they need their own system. Also some libraries just arrange books by their accession if they are in a closed area and have to be paged. NYPL is one example of this. You can’t go over to the 5th Avenue branch and browse.