Mixing Major and Minor Pentatonic

I believe you're thinking about if a song is in a major key you can mix the major and minor pentatonics. That's done a lot. If the song is in a minor key, this doesn't work so well.

It's really the other way around - if a song is in a major key, you can use the major or minor pentatonic... but if it's in a minor key, the major doesn't work so well. Putting a b3 in a scale against a 3 in a chord makes it sound 'bluesy', but putting a 3 in a scale against a b3 in a chord tends to sound wrong.

But that's just using pentatonic scales, and there are 7 other notes. So... on to the specifics.

There's nothing wrong with using D in a C minor scale. It's part of every minor scale type - natural, harmonic, melodic, dorian. It'll sound ok against the Cm, because it'll make a Cm(add9) against it.

There's not much wrong with A either. In the C natural minor scale, A is flat... but in the melodic minor or Dorian it's natural. Since there's no A in a Cm chord, you'll get a 'jazzy' effect using it - jazz often uses these scales over minor chords.

So I'd narrow strategy #1 and just avoid the E note.

Nothing wrong with strategy #2 either, although you'll probably want to expand your tones a bit. Play the E, but pull it off to an Eb. The D and A will still sound fine.

Strategy #3 will work too... but technically you'll be bending the A up to Bb rather than A# (Cm is a flat key, relative to Eb)

As to why certain notes sound good... well, part of it is cultural experience. We're used to hearing certain sounds, so the familiar tends to sound better than the unfamiliar. Some folks will tell you it's about frequency consonance - that 'good' notes like ratio like 3:2 (perfect fifth) or 4:3 (perfect fourth), so the vibrations are a near match, while 'bad' notes like a minor ninth have strange relationships like 31:15. There's a grain of truth to that, but the fact is ALL our notes are 'out of tune' except the octave - so it's more true in theory than practice.

Pythagoran theory says if A is 440, E will be 660 - a 3:2 ratio. In reality, we put E at 659.25, a hair flat. The major second should be 9:8, so that's B495; we play B493.88. Some intervals are off more than others, but none are dead on, and they haven't been for a long time - I think it's mostly the fact that 'good' sounds are simply ingrained in us by experience.

Thanks guys, makes a lot of sense.

Vic