G major blues or rock and roll backing track but playing G minor pentatonic or a mix of minor or major , why does this work? I also noticed the E flat which is not part of the pentatonic scales just the natural G minor scale this note also sound nice, I noticed it only seems to work with blues or rock and roll not mainstream Rock or metal just wonder if anyone can explain the theory behind it ?
It's all about tension and resolution. When you mix those minor and blues scales with major scales, you create tension with the notes exclusive to minor and blues scales, and then you resolve it with the notes of the major scale.

It's a textbook example of creating and resolving tension really: the simplest resolution is achieved when you play a note and follow it with a note that is a semitone higher. Then you have just created and resolved tension. So the notes A# and D# of G minor create tension, and H and E of G major resolve it for one example.

Of course this just scratches the surface. You should read up a bit about tension/resolution, as tonal music is pretty much based on it. It could help you understand borrowed tones and substitute chords better.
Last edited by guitar/bass95 at Feb 20, 2015,
Blues mixes major and minor. Why minor pentatonic works over a major blues track is because it just sounds bluesy. We are used to the sound - it's the sound of the blues. Some of the notes are a bit dissonant (actually just the third over the I chord and the 7th of the scale over the V chord - they are the only real clash notes). But if you want to stick with one scale over all chords, the minor pentatonic will work pretty well over everything (in basic 12 bar blues).

If you don't want to sound bluesy, don't use the minor pentatonic scale over a major key song. Use the major pentatonic or major scale instead (though of course if you are playing over a bluesy backing track, nothing is going to make it sound not bluesy). But you need to look at the chord tones.

G major blues is G7 (G B D F), C7 (C E G Bb) and D7 (D F# A C). So if you want everything to work over the chords perfectly, emphasize the chord tones. No one scale will work over it perfectly (notice how the chords have both F and F#, and Bb and B in them), but the G minor pentatonic will work pretty well over it. So maybe use that as your base scale and add some chord tones to it when needed. Oh, and of course adding the b5 (in this case Db) will sound really bluesy. It's not part of any of the chords but sounds great when used before a consonant note a half step higher or lower than it (so if the next note you are going to play is D or C, playing a Db just before it sounds good). But spending too much time on the note/not releasing the tension the note creates may not sound good.

Adding other notes is also possible. You can actually use all the 12 notes, you just need to know how to use them. For example using an Eb may sound good when you play an E or a D after it, just like any note that is not part of the scale will if you just release the tension by going a semitone up or down to a consonant note. That's the safest way to use them.
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I would consider Eb a particularly dissonant note over G major blues but to each his own! As MM said, you can use any of the 12 notes if you have a decent exit strategy!

Eb is the b9 of D7 and resolves down to the to the 5 of G.

It's also the #9 of C7 which resolves to the root C or major third E.