At first, I thought that the choices for the "3 'Principal' Chords" seemed odd. I'm a struggling guitar player (for over 40 years now!) and I never thought of the 7th chords as 'Principal'. In 10 minutes I found myself experimenting with chords I hadn't used in years - that alone made it worth the price! I have plenty of chord books around, but none of them are nearly so handy as having "principal". minor and "additional" chords for every key in a simple laminated page that can be taken anywhere.