MODULATION is a process which results in a shift of tonal center or a KEY CHANGE.
There are three types of modulation. Most modulations occur between closely related keys which are those keys that differ by no more than one accidental in the key signature and their relative minors. (see chart)
COMMON CHORD MODULATION:
A common chord is one that is common to two keys offering a smooth transition from the first to the next. The spelling of the chord is the same in both keys. This common chord is called a Pivot Chord and the analysis looks like this:
Where can Common Chords occur? Create a chart of both keys.
A B C D E F G
F G A B C D E
D E F G A B C
ii iii IV V vi vii 0 I
V vi vii 0 I ii iii IV