A Bit of Music Theory

Explanation:

1. For C Maj, 1=C, 4=F, 5=G; Similarly, 1=D, 4=G, 5=A; and so on;

2. the overlapping key (e.g. C/Am) on the little finger is the corresponding relative minor; this works for C/Am, D/Bm, F/Dm, G/Em; but not quite (add #) for E/C#m, A/F#m, B/G#m;

3. For 1=C, the common chords are: C, Am, Em, F, G7 (for 1, 3, 4, 5); and so on.