A couple of basic chords are the open string C chord and G7 chord. Reviewing the rules with these two chords are as follows.
C and G7 chords. Learn to set the fingers as shown in the digram below.
Now if it is done in slow motion, the lowest 2 notes in the C chord, have the same shape as the lower 2 notes in the G7 chord. Note the shapes below. This is the finger relationship rule. 2nd and 3rd fingers keep the same shape when changing from the C chord to the G7 chord. They move as a unit together.
The first finger of the C chord, on the second string, first fret, moves to the first finger of the G7 chord, on the first string., first fret. This is the Finger relationship rule.