Watch the Thirds online guitar lesson by Robben Ford from Chord Revolution: Foundations
We are looking at a G major triad and the scale that runs through it. We've learned that we can harmonize that major scale, and we have all these chord voicings within the context of a major scale. They have names. Tonic, two chord (second chord), three chord, four chord, five chord, six chord, seven chord, one chord. So we have G major scale, all these wonderful triads. Now we don't have to play all three notes of the triad at once to define the chord. We're not locked into it. I want to talk to you a little bit about thirds. It's referred to as thirds because we're playing tonic and third, and we keep that relationship between those two notes.
Throughout the major scale you can play thirds. I'm just playing a vamp, G to E minor. There is so much you can do with just those two notes. Also, basically, with just a G major scale. Everything is flat out G major and you want to learn how to play these all over the guitar. This is something that should not be difficult for you - To learn how to play these little third voicings all over the guitar. There are certain things you really need to know how to do. You should really learn how to do this. Just like you should learn how to play the pentatonic scale all over the guitar, just walking all over the guitar. You really need to know about thirds. They are a simplified version of a triad. It's the same activity, just broken down.