Rule of the Common Tone.
Using the C and Am Chords. The 1st finger on the 2nd string and the 2nd finger on the 4th string are common tones. Both are used in the C and Am chords. First the Chord showing only the fingers held down, and then with the open strings added
Two other chords with Rule of the Common Tone involved.
Changing from the Dm chord to the G7. The first finger used on the first string is a common tone used with the Dm and G7 chord.