Every so often, I think about the minor pentatonic.
Yes. I'm that kind of guy. The guy who, out of the blue, thinks about scales.
For the longest time, I thought about the pentatonic as a subtractive thing. Let me explain.
You have the first, fourth and fifth right? Assume A minor, so I have to type # and b far less. The root is A, and the A minor scale is A B C D E F G. The fourth is D, and D minor is D E F G A Bb C. E is the fifth, and E minor is E F# G A B C D. So, speaking from the root, there's disagreement about the second and the sixth, the B/Bb and the F/F#. So, we use what everything agrees on and subtract the second and the sixth, so we now have the pentatonic minor, A C D E G.
This is useful for a few reasons. If you subtracted it and you know why, you can always add it back. If you're on the turnaround, you can hit that B for the E and drop it back to a Bb for the D, then one more to A for the root.
But there's things it can't explain, like why the blue third, the "Spoonful" third, isn't really either C or C#.
Steve Kimock can explain. And it's all adding harmonics.
The harmonic relationship between the root and the fifth is considered perfect. That's what they call it in theory books. And because the root is fifth to the fourth, that's also a perfect relationship. Thus the I IV V. And that's three of five. Where are the other two?
We're switching to E minor pent, so tune up the Es on your guitar. There's a harmonic just back from the G#. Hit it, and then try to find it with your guitar. Here's a hint: bring your slide. Because it isn't the G#, but it's far sharper than G. The blue third is not the blue because "it's happier than the minor third but sadder than the major third." It's the blue third because the design of the universe made that the harmonic.
This leaves the flat seventh. Find the harmonic just behind the G. The D on the B string will sound sharp. Again, the flat seventh we find is not the one that occurs in nature. Or is it the sharp six, really?
But anyway, that's the additive way to the pentatonic minor. If we adjust to where we can get with our frets, that's our E G A B D. Or, back to A, the A C D E G.
The practical application of this information is left as an exercise for the reader.