You may be wondering why I listed the minor chords with the “vi” first. If you look at the circle diagram, do you see how the “I, IV, and V” chords form a triangle with the “I” chord at the pointy end? Notice how the minor chords forms the same triangle shape, with the “vi” chord at the pointy end. Notice that each minor chord is exactly 3 half-steps down from a major chord. Each minor chord corresponds to a major chord:
“vi” and “I”
“ii” and “IV”
“iii” and “V”
This concept is called the relative minor or relative major. The “vi” chord is the relative minor of the “I” chord. The “I” chord is the relative major of the “vi” chord. The “ii” and “IV” have the same relationship, as do the “iii” and “V” chords.
This makes it easier than you might expect to understand key structure, since we basically just have a triangle of 3 major chords, and another triangle of 3 minor chords that is essentially a mirror image of the first triangle. Because of this simple geometry, we can use a few simple tricks to map out any major or minor key on the guitar.