The inside row of counter bases represent the 3rd note of the scale so E is inside C, B is inside G etc. If you play a note from the second column, the note inside it and the note above it, you are playing the major chord of the scale. Interestingly if you hold down these three buttons, the button inside on the third row will automatically also go down, because this is the button that plays the same major chord. The same is true for any chord, if you hold down the constituent notes the chord button presses in automatically.

I learned to play a P.A years ago, but being a gentleman I don't do so any more. It was a great way of learning about chord structures, related chords and other parts of musical theory which has stood to me in learning to play other instruments over the years.