Watch the Harmonizing the Major Scale online guitar lesson by Robben Ford from Chord Revolution: Foundations
I want to demonstrate a little more on the relative minor triad to the major triad of the G major scale. So we're still in G. Major triads in this key being G, C, and D. The other triads in the scale are minor, A minor, B minor, and E minor. So three major triads, three minor triads and the seventh one being a diminished triad. So the relationship between G major and E minor is very obvious, and especially if you play this open G on the guitar, E minor. Virtually the same chord. And with a different root note, being the sixth degree of the G major scale, the C. The minor triad relative to it is A minor, down a minor third, D, B minor. Then again, we have the diminished chord going into G.
So these things are what most songs are made of. Especially if you listen to older rock n roll, you really hear the flat out use of those chords exactly. Very little variance in keys. Keys don't change so much, and so you're basically using that information in most music. You're just in different keys, but the chords all have the same relationship. The scale is one major scale. There's so much that can be done with just this information. These triads are the blues chords - I, IV, V and their relative minor chords. So you can play most songs with just that information, and one of the classic examples, the best example I can think of, that really just demonstrates the use of harmonization of the major scale - Bob Dylan's "Like A Rolling Stone" is virtually just the harmonization of the G major scale. Then he goes down after this little bit, right down the scale. Then it's the major triads of the major scale.