Watch the Relative Minor online guitar lesson by Robben Ford from Chord Revolution: Foundations
We're going to move to the key of G, and the reason being, once again, it's a very friendly key on the guitar. You could use basic triads very easily in this key. I play a G major using open strings. This is the way it's generally taught, but I find that a little awkward. The way I play it allows me to move to other chords a little easier. And also you don't necessarily need to play the third there. Because you're duplicating it up here, and it sounds better up there, anyway. A little further away from the root of the G. To do this, it's a little heavy. Basically I would play it like that and I'm damping the A string with this finger. Easy to get to E minor with that. You don't have to do this. We're in the key of G. As demonstrated before, playing the E major harmonized scale, here is your basic 1st position G triad when not using open strings. I very rarely play bar chords, I'll play this. Sometimes I will play that low G and sometimes not - so key of G. Let's harmonize the G major scale. Root chord - Second chord or two chord, which is, in this case, A minor. Three chord, four chord, five chord, six chord, seven chord, and the octave. So now you know, if somebody says, "let's do a I-IV-V thing, a blues". The key of G, so one chord, four chord, five chord. Now you know.
These triads - One chord, two chord, three chord, four chord, five chord. Let's just stop right there. So you've probably heard reference to what is known as the relative minor. Within the context of a harmonized major scale there are three major triads. There's G major. Your two chord is a minor chord, A minor. Your three chord is a minor chord. Again we're in the key of G. B minor. Four chord is a major triad. Five chord is a major triad. E minor. F# diminished. G major triad. So G, C, and D are your three major triads in the key of G. Very conveniently on the guitar, and I think it's one of the reasons (if not the main reason) why just playing major chords and using the open strings exists, is because these relative minors are very handy. Key of G. Standard open G voicing. The relative minor is E minor. Basically because they sound identical, save for the fact that you are putting a different root underneath that G major triad. It's a minor third below. You could also refer to it as a sixth above. It's either a minor third below, G with an E below. If you were to play it like that, that'd be your six chord in a literal sense. Just moving up the scale - One, two, three, four, five, six. So G major, E minor is the relative minor.
The C major triad, minor third below in the root, gives you A minor which is the three chord in the key of G. The third major triad is D. Minor third below in the root gives you B minor 7. So G, E minor. C, A minor. D, B minor. Now again, the point being, these chords sound almost identical. Just by putting a different root below the triad, you get a different sound. You get a different feeling. You get a different application of those same notes of the major scale. The fingers don't even change. The voicing doesn't change. Just the root note changes. It's a little different when you hit the D chord and put minor third in the root. This is your standard jazz voicing of a B minor 7 chord. Visually, it's the same chord, just with a different root. We're already moving into a broader way of viewing the major scale and the basic triads that exist in a harmonized major scale.