Okay, now this applies to guitar mostly, but it probably works on other string instruments, and you can apply the basic principles to almost any instrument. So, first off, I'll assume the guitarists know the CAGED system, but for the others, I'll give a very brief explanation.

On a guitar, there are several enharmonic notes. I can play a C3 on the third fret of the A string (assuming standard tuning) or on the eighth fret of the low E string. So a C major chord can be played either as
___o_o
||||o|
||o|||
|o||||
C-form

or

______
||||||
||||||
|o|||o (3rd fret)
||||||
||ooo|
A-form

or


o|||oo (8th fret)
|||o||
|oo|||
E-form

or even


||ooo|
||||||
|o||||
o||||o <--(8th fret)
G-form

And the D-form would an octave higher, but here:


||o|||| 10-th fret
|||||||
||||o|o
|||||o|
D-form

So the CAGED system builds "boxes" around those chord shapes, applying them to the pentatonic, diatonic, and harmonic minor scales. It's very helpful, because it facilitates thinking in scales and chords the same time. But here's the cool thing for this article. First, look at the pentatonic boxes.


       oooooo   |ooo||   |||o||   oooo|o   ||oo||
       ||||||   o|||oo   ooo|oo   ||||o|   oo||oo
       |ooo||   |||o||   ||||||   ||oo||   ||||||
       o|||oo   ooo|oo   oooo|o   oo||oo   oooooo
       ||||||   ||||||   ||||o|   ||||||   ||||||
major: G-form   E-form   D-form   C-form   A-form
minor: E-form   D-form   C-form   A-form   G-form

Okay, and these are the diatonic boxes.


       |||o||   ||||||   ||oo||   ||||||   |ooo||
       oooooo   oooo|o   oooooo   oooooo   ooo|oo
       ||||o|   oo||oo   ||||||   o|||oo   |||o||
       oooo|o   ||oo||   oooooo   |ooo||   oooooo
       oo||oo   oooooo   o|||oo   ooo|oo   ||||o|
major: G-form   E-form   D-form   C-form   A-form
minor: E-form   D-form   C-form   A-form   G-form

(Note: sorry if these are small and hard to read, but there should be plenty of nice big pictures out there on the Internets.)

Obviously the same pentatonic forms and diatonic forms work together. But if you look, you'll notice that more than one pentatonic box can actually fit over a given diatonic box. The E-form pentatonic box fits over not only the E diatonic, but also the C and A diatonic boxes. The C-form pentatonic box fits C, E, and G. The C diatonic box could be played with the G pentatonic or the E pentatonic. And of course you can use a small fragment of any pentatonic box with any diatonic box.

To try and clarify, if I play the E-form pentatonic box at the fifth fret, I may be playing over A minor, E minor, or D minor (or any of their modes).

Yeah, you may be thinking "so what? That's just a ridiculously circuitous way of saying you can play a G minor chord over C minor." Well, yeah. It sort of is. But this way, you don't have to think about chords if you know your scales much better. Just run through a certain pentatonic, say the E-form. Let's say you're playing in E-minor at the seventh fret of the A string. To play the root note there, you could use the C-form or the A-form. If you start with the E-form pentatonic in that position, throwing in an F# is a bit of a surprise. People would expect an F, because this box in this position would normally be played over A minor or C major, so the F would either be a minor sixth or perfect fourth, respectively. So you can start in one of those scales and then segue seamlessly into E-minor, or more likely a major mode of it. Or you could really do almost anything you want.

Another neat trick I've just come up with is playing several pentatonics over the same scale. Playing in E-minor, same place, you could descend with the E shape first, then the G shape, then whatever. Here's what I've got in tab:


8p5---------7p5---------5p3---------------
---8p5---------7p5---------7p5------------
------7p5---------7p4---------5p3---------
---------7p5---------7p4---------7p5-4----
------------------------------------------
------------------------------------------

For non-guitarists, that would be C A G E D C A G, B A F# E D B A F#, A G F# E E D D C B. Sorry, I don't know how to put an actual score in text, but hey, better than nothing, right?

Have fun!