COMMON CHORD MODULATIONS

A common chord modulation contains chords that can be expressed in both the old and new keys, as illustrated in the next figure. Each of these common chords must be, in both the old and new keys, one of the following: 

• A diatonic chord 

• One of common types of altered chords (like Neapolitan chord or Augmented sixth chord)

Figure: Common chord modulation

Annotating common chord

If we want to annotate the fact that a particular modulation is a common chord modulation, we can do so by annotating the common chord. Usually we can find more than one common chord and annotate all of them but there is no need for that. It is enough to annotate only one common chord, the one at the location of the modulation. A common chord is annotated in both keys. 

The next figure shows how to annotate a common chord. 

Figure: Annotating common chord, first in the new key and then in the old key

Alternatively, the common chord can be annotated like in the next figure.

Figure: Annotating common chord, first in the old key and then in the new key

The first figure visually emphasizes the chord progression in the new key that begins with the fourth chord, while the second figure visually emphasizes the chord progression in the old key that ends with the fourth chord. This is useful when one of these two chord progressions is more convincing than the other one and we want to visually emphasize this fact. For instance, when one of these two chord progressions has an obvious P > D > T form. 

In the chapters that follow, we shall see many examples of common chords.