Single letter tags?

I have to ask: what's up with all the single letter tags? Like a, b, d (skipped 'c' because, well, that one makes more sense). I first ran across p and am still mystified about that one. It doesn't seem to be anything about alphabetization.

I think some people are using them for the Library of Congress classification system. (I note works tagged P are also tagged PN or PR.)

Ooh. Thank you, that makes sense.

P - language and literature - PR is English literature (mostly British but not all), PS is American, PT Germanic, and so on.

I use these, as it's quite handy to be able to see which reference groups are well-represented and which are not (my cookery and craft sections way outnumber science, which I should do something about!). When capital letter tags were sorted separately from lower case, it also made a very useful image in the tag cloud, which doesn't happen any more. :(

BoPeep, I apologize for the idiotic question, but why "P" for literature?

Not a clue. It's the LoC classification system, not mine. ;-) The link Slothman gave lists them all, but no explanation as to why they're in the order they are.

Re >5 aluvalibri: and >6 BoPeep:

The letters in the LoC classification are not intended to provide an alphabetical arrangement of main class but to arrange them in a conceptual order, as do the numbers in the Dewey classification. (Leaving aside the question of how good or bad one might think either system's conceptualizations are!) LoC just starts off with letters. It's just a coincidence when the initial class letter actually stands for what the class is about, e.g. M - Music