20th Century compositional practices

I had an adult piano student email me with questions about analyzing
20th century works;  he was trying to use major/minor tonality with
composers who used avant guard techniques.   He has not studied 20th
century techniques yet, though his questions are good ones, if
premature.

I tried to respond, and have put this response in a Q & A section on
the Theory page at:

http://beststudentviolins.com/TheoryNotes.html#QA

Problem is, I can't recall what the seven categories are which Virgil
Thomson used to categorize 20th century music.  I keep losing this
information and now I can't find it.  Does anybody know what Thomson's
seven categories were?

Please comment on the other notes, as well, if you would.

Thanks in advance,
Connie