It's been a while since I did anything on my series about library ways of knowing. If you'd like to refresh your memory:
Today I'll finish my discussion of classification, and distinguish it from subject analysis, since that distinction often seems to confuse, especially in our digital age.
So if we'll recall, the goal we set for ourselves was to collocate physical books on shelves in such fashion that their arrangement would be useful to information-seekers. With most non-fiction, that means collocation by subject, by what the books are about.
(There are lengthy philosophical discussions of "aboutness" in the information science literature. I recommend avoiding them with all your strength. They make my eyes bleed.)
To make this work, we have to map knowledge-space onto physical space: divide up human knowledge into convenient slots to assign books to. This is, you might say, a tall order: an ontology of infinite domain, but where each item can only fit in one place.
In the States, most libraries use one of two such maps: the Dewey Decimal System or the Library of Congress Classification. About the kindest thing one can say for Dewey Decimal is that it was a product of its peculiar time; for today's purposes, it is heavily overnumbered in religion, for example, and undernumbered in science. Perhaps worse, its sense of the world is not exactly immediately intuitive to the modern eye: why the long separation of geography from the so-called "social sciences," of which psychology is apparently not one?
This is one danger of any would-be universal classification. Our sense of the world and its knowledge changes over time, sometimes quite a lot and quite suddenly. If our ontology doesn't keep up, it serves its purposes less and less well. How easy is it, really, to find the right shelf in a library of any size organized by Dewey Decimal? Considerations such as these no doubt informed the shift of one library (and later others) to the BISAC codes typically found in large bookstores.
Another danger of the universal classification is that its specificity is of necessity somewhat limited. Many medical libraries, for example, ditch Library of Congress Classification because it just doesn't drill down far enough into medical minutiae for their needs. The NLM Classification fills the gap.
With physical books, we cannot escape the constraint that each book must go in one and only one place on the shelf. Once we're away from the physical item, that constraint disappears. The card catalogue was the first desperately clever escape from the tyranny of the physical item: in a card catalogue, the same book could be "shelved" by author, title, and one or more (usually three to five, to avoid overproliferation of cards) subjects assigned to it by the cataloguer.
This meant the addition of a subject-heading system to the classification vocabulary. You can't just add more classification numbers to the physical item; you then imply that it goes in more than one place! This is the difference between Library of Congress Classification and Library of Congress Subject Headings. Under most circumstances, the LCC number assigned to a book will correspond closely in meaning to the first LCSH assigned in the book's catalogue record. They are still distinct systems, however! Don't confuse them. Librarians chuckle behind their hands.
Of course, digital items don't have to live in just one space. Classification is therefore slowly giving way to subject analysis and similar ways of relating items to each other as digital libraries develop.
And that, in a remarkably simplified nutshell, is how books are arranged on shelves in libraries. It doesn't happen by magic!