I discovered something fascinating. When you construct 7th chords with major and minor thirds, you get seven possibilities: dim7, m7b5, m7, m/maj7, 7, maj7, aug7. See here for all the combinations:

m3 m3 m3 - 1 b3 b5 bb7 - dim7
m3 m3 M3 - 1 b3 b5 b7 - m7b5
m3 M3 m3 - 1 b3 5 b7 - m7
m3 M3 M3 - 1 b3 5 7 - m/maj7
M3 m3 m3 - 1 3 5 b7 - 7
M3 m3 M3 - 1 3 5 7 - maj7
M3 M3 m3 - 1 3 #5 7 - aug7
M3 M3 M3 - 1 3 #5 #7 - (aug triad plus octave, not a 7th chord)

I was writing out chord scales for melodic minor and harmonic minor the other day. I've done this before, but I don't recall noting that the harmonic minor has all seven of the above 7th chords in it.

Looking at each mode of the harmonic minor scale, here are the 7th chords built on each one:

1 2 b3 4 5 b6 7 - m/maj7
1 b2 b3 4 b5 6 b7 - m7b5
1 2 3 4 #5 6 7 - aug7
1 2 b3 #4 5 6 b7 - m7
1 b2 3 4 5 b6 b7 - 7
1 #2 3 #4 5 6 7 - maj7
1 b2 b3 b4 b5 b6 bb7 - dim7

I think that's cool. It always bugs me when parts of music theory involve arbitrary memorization without logical patterns. I often feel like the whole foundation is unsound. When I see something like this, it brightens my day and puts a twinkle in my eye. Then I read the news.