Each chord, rotating clockwise, is a perfect 5th apart. The outer circle is filled with Major chords. The inner circle holds the Major chord's relative minor, which is very important in ad-libbing (playing solos). The center shows the key signature (sharps and flats) for both the major its' relative minor.
A major chord and its relative minor share the same key signature (number of sharps and flats). The relative minor chord of any major chord is the triad built on the sixth note of the major chord's scale. For example, in the key of C Major there are no sharps or flats. Counting up from C, the sixth note of the scale is A. Therefore, the relative minor chord for C Major (C E G) is A minor (A C E).
The relative minor is an especially useful chord for writing the break to a song. If you have a tune with a verse and chorus but need to break it up in the middle, try changing the feel and moving to your relative minor.
Use this chart to find the relative minor to any Major key and vice versa. For a quick 3 chord song, chose any Major chord (I, Tonic), pick the chord to its left (IV, Sub Dominant) and the chord to its right (V, Dominant). You now have the basis for a I - IV - V blues progression in any key! This progression is one of the most often used in Rock Music.