While I was in the stacks of a public library yesterday (in a larger city than my city), I noticed that the music section was divided between what would be termed "classical music," and a mishmash of what would be loosely termed "non-classical music." Books about jazz were interspersed with books about blues and books about non-western (another broad classification, I know) music. The "non-classical" section was about five times larger than the "classical" section. This is a public library, and it follows the Dewey Decimal System, which organizes a library's collection of books about music (as opposed to actual printed music) under the numbers 780 and 781.
The Library of Congress organizes its books about music this way:
In the 19th century, when these systems were created (Dewey and LOC ), nobody had any idea that books about "popular music" (ML 3469-3541), "dance music" (ML 3400-3465), and "folk, national or ethnic music" (ML 3544-3776) or Dewey's "kinds of music" (781.5), and "traditions of music" (781.6), would outnumber books about the rest of the field.
And what's up with Dewey's 780.8? What does he mean by "kinds of persons?"