ENHARMONIC COMMON CHORD MODULATIONS

In the following chapters we examine common chord modulations which are also enharmonic modulations. We call such modulations enharmonic common chord modulations.

In enharmonic common chord modulations we find common chords that are enharmonically reinterpreted in one of the keys. We call such common chords enharmonically reinterpreted common chords. 

The material that follows is advanced. The reader should have a solid understanding of the enharmonic equivalence, common chord modulation, chords, chord positions, chord roots and keys. However, all the required knowledge was already presented in the book.  

Enharmonic reinterpretation of German sixth chord

The German sixth chord in the first inversion can be enharmonically reinterpreted as the dominant seventh chord in the root position.

The next figure shows how the German sixth chord in the first inversion, in the C major key, can be enharmonically reinterpreted as the dominant seventh chord in the root position, in the Db major key. This is done by enharmonically reinterpreting F# as Gb which also changes the chord root from F# to Ab. We see that such an enharmonic reinterpretation provides an opportunity for modulating from the C major key to the Db major key. Generally, a modulation is possible by the ascending minor second (if we reinterpret the German sixth chord as the dominant seventh chord) or by the descending minor second (if we do the opposite i.e. reinterpret the dominant seventh chord as the German sixth chord).

Figure: The German sixth chord and the enharmonically equivalent dominant seventh chord

The next figure shows an example of enharmonic common chord modulation based on the enharmonic reinterpretation of the German sixth chord. The modulation is from Chopin’s Nocturne Op. 9 No. 1 in Bb minor. The figure shows the beginning of the middle part of the nocturne. While the beginning and end of the nocturne are written in the Bb minor key, the middle part is written in its relative key: Db major key. 

Figure: The enharmonic common chord modulation in Chopin’s Nocturne Op. 9 No. 1 in Bb major

We see that the common chord is the chord A7/C#. If we enharmonically respell this chord from C# – E - G – A to Db – Fb – G – Bbb (or as a stack of thirds G – Bbb – Db – Fb) we get the German sixth chord in the Db major key. Thus:

1.       The common chord is an altered chord in one of the keys 

2.       The enharmonic reinterpretation is necessary in one of the keys

From this we conclude that the examined modulation is enharmonic altered common chord modulation.

Enharmonic reinterpretation of diminished seventh chord

The diminished seventh chord can be enharmonically reinterpreted as the diminished seventh chord with a different root.

In chapter Diatonic seventh chords in minor keys, we learned that the diminished seventh chord occurs in a minor key as the seventh chord on the raised seventh scale degree. In this chapter, we shall explore three different possibilities for enharmonically reinterpreting the diminished seventh chord, which can be used in modulations between the minor keys. In fact, they can be also used in modulations between the major keys, if borrowed chords are used. 

The next figure shows how the diminished seventh chord in the first inversion, on the raised seventh degree of the A minor key, can be enharmonically reinterpreted as the diminished seventh chord in the root position, on the raised seventh degree of the C minor key. This is done by enharmonically reinterpreting G# as Ab which also changes the chord root from G# to B. We see that such an enharmonic reinterpretation provides the opportunity to modulate from the A minor key to the C minor key. Generally, a modulation is possible by the ascending minor third (if we reinterpret the diminished seventh chord in the first inversion as the diminished seventh chord in the root position) or by the descending minor third (if we do the opposite i.e. reinterpret the diminished seventh chord in the root position as the diminished seventh chord in the first inversion).

Figure: The diminished seventh chord and the enharmonically equivalent one in the different key

The next figure shows how the diminished seventh chord in the second inversion, on the raised seventh degree of the A minor key, can be enharmonically reinterpreted as the diminished seventh chord in the root position, on the raised seventh degree of the Eb minor key. This is done by enharmonically reinterpreting G# as Ab and B as Cb which also changes the chord root from G# to D. We see that such an enharmonic reinterpretation provides the opportunity for modulating from the A minor key to the Eb minor key. Generally, a modulation is possible by ascending diminished fifth (if we reinterpret the diminished seventh chord in the second inversion as the diminished seventh chord in the root position) or by the diminished fifth down (if we do the opposite i.e. reinterpret  the diminished seventh chord in the root position as the diminished seventh chord in the second inversion).

Figure: The diminished seventh chord and the enharmonically equivalent one in the different key

The next figure shows how the diminished seventh chord in the third inversion, on the raised seventh degree of the A minor key, can be enharmonically reinterpreted as the diminished seventh chord in the root position, on raised seventh degree of F# minor. This is done by enharmonically reinterpreting F as E#, which also changes the chord root from G# to E#. We can see that such an enharmonic reinterpretation provides the opportunity for modulating from the A minor key to the F# minor key. Generally, a modulation is possible by the ascending major sixth (if we reinterpret the diminished seventh chord in the third inversion as the diminished seventh chord in the root position) or by the descending major sixth (if we do the opposite i.e. reinterpret the diminished seventh chord in the root position as the diminished seventh chord in the third inversion).

Figure: The diminished seventh chord and the enharmonically equivalent one in the different key

The next figure shows an example of enharmonic common chord modulation based on the enharmonic reinterpretation of the diminished seventh chord. The modulation is again from Chopin’s Nocturne Op. 9 No. 1 in Bb minor. In the previous example, we examined the modulation from the Db major key to the D major key in the middle part of the nocturne. In the next figure we see that soon after the first modulation, the nocturne modulates again to return to the Db major key. 

Figure: Another enharmonic common chord modulation in Chopin’s Nocturne Op. 9 No. 1 in Bb major

We see that the common chord is the chord C#o7/G. It does not appear in the minor key but in the major key, as a borrowed chord. If we enharmonically respell this chord from G - Bb – C# – E to G – Bb – Db – Fb, we get Go7 or viio7 in the Ab major key (again a borrowed chord). This corresponds to the second case of the enharmonic reinterpretation of the diminished seventh chord which provides an opportunity for modulating by the ascending diminished fifth: from the tonic D to the tonic Ab, in this case. However, the situation is even more complex because Go7 is used as a secondary leading-tone chord, thus the modulation is to Db major key and not to the Ab major key. We can conclude that this modulation is enharmonic altered common chord modulation. This is a good example how a secondary chord can appear in altered common chord modulation. 

Please note, that in a modulation to the closely related key, practically every chord in one key can be interpreted as a secondary chord in the other key so this is not a good case for usage of secondary chords as an approach to identify altered common chords. 

Enharmonic reinterpretation of French sixth chord

The French sixth chord in the second inversion can be enharmonically reinterpreted as the French sixth chord in the root position.

The next figure shows how the French sixth chord in the second inversion, in the C major key, can be enharmonically reinterpreted as the French sixth chord in the root position, in the F# major key. This is done by enharmonically reinterpreting Ab as G# and C as B#, which also changes the chord root from D to G#. We see that such an enharmonic reinterpretation gives an opportunity to modulate from the C major key to the F# major key. Generally, a modulation is possible by the ascending augmented fourth (if we reinterpret the French sixth chord in the second inversion as the French sixth chord in the root position) or by the descending augmented fourth (if we do the opposite i.e. reinterpret the French sixth chord in the root position as the French sixth chord in the second inversion). 

An interesting fact is that both this cases lead to enharmonically equivalent keys. For instance, the modulation from the C major key to the F# major key and the modulation from the C major key to the Gb major key both leads to the enharmonically equivalent keys: the F# major key and Gb major key (the reason being the augmented fourth and its inversion are enharmonically equivalent).

Figure: The French sixth chord and the enharmonically equivalent one in the different key

Enharmonic reinterpretation of augmented triad chord

The augmented triad chord can be enharmonically reinterpreted as the augmented triad chord with a different root.

In chapter Diatonic triad chords in minor keys, we learned that the augmented triad chord occurs in a minor key as the triad chord on the third degree when the seventh degree of the minor key is raised. In this chapter, we shall explore the possibility for enharmonically reinterpreting the augmented triad chord, which can be used in modulations between the minor keys.

The next figure shows how the augmented triad chord in the first inversion, on the third degree of the A minor key, can be enharmonically reinterpreted as the augmented triad chord in the root position, on the third degree of the C# minor key. This is done by enharmonically reinterpreting C as B#, which also changes the chord root from C to E. We see that such an enharmonic reinterpretation provides an opportunity for modulating from the A minor key to the C# minor key. Generally, a modulation is possible by the ascending major third (if we reinterpret the augmented triad chord in the first inversion as the augmented triad chord in the root position) or by the descending major third (if we do the opposite i.e. reinterpret the augmented triad chord in the root position as the augmented triad chord in the first inversion). 

Figure: The augmented triad chord and the enharmonically equivalent one in the different key

The next figure shows how the augmented triad chord in the second inversion, on the third degree of the A minor key can be enharmonically reinterpreted as the augmented triad chord in the root position, on the third degree of the F minor key. This is done by enharmonically reinterpreting G# as Ab, which also changes the chord root from C to Ab. We see that such an enharmonic reinterpretation provides the opportunity for modulating from the A minor key to the F minor key. Generally, a modulation is possible by the ascending minor sixth (if we reinterpret the augmented triad chord in the second inversion as the augmented triad chord in the root position) or by the descending minor sixth (if we do the opposite i.e. reinterpret the augmented triad chord in the root position as the augmented triad chord in the second inversion).

Figure: The augmented triad chord and the enharmonically equivalent one in the different key