Enharmonic modulation

The most frequently used enharmonic devices:

A. Enharmonic reinterpretation of a diminished seventh chord.

B. Lowering one note of a diminished seventh chord by a half step to create a dominant seventh chord.

C. Enharmonic reinterpretation of a Ger⁶ chord as a dominant seventh chord.

D. Enharmonic reinterpretation of a Fr⁶ chord into a root position French chord in a major/minor key a tritone away.

A. Enharmonic reinterpretation of a diminished seventh chord

The diminished seventh chord equally subdivides the octave, with every interval in the chord being a minor third. Because of this equal subdivision of the octave, any diminished seventh chord can be enharmonically spelled four different ways.

Depending on the enharmonic spelling, a diminished seventh chord thus can resolve to three other pairs of parallel major or minor keys. A single diminished seventh chord can thus resolve to the tonic in eight different keys:

In the example below, the B^o7/F chord in C major is reinterpreted as B^o7/D in E major.

B. Changing a diminished seventh chord into a dominant seventh chord

The diminished seventh chord is closely related to the dominant seventh chord. Any one of the four notes of a vii^o7 chord can be lowered by a half step to create a V⁷ or V⁷ inversion. In reverse, raise the root of any V⁷ or secondary dominant V⁷ by a half step to create a vii^o7 chord.

The example below shows a reinterpreted secondary diminished seventh chord altered into a dominant seventh chord of the new key.

*C ^o7 = E^o7/D and then changed into E⁷/D

In the example below, the B^o7/A chord in C minor is considered to be E^o7/G, which is the leading tone seventh chord of F major. The chord is then altered into a dominant seventh chord inversion in the new key.

C. Enharmonic interpretation of a German sixth chord
as dominant seventh chord

A German sixth chord becomes a dominant seventh chord by the enharmonic respelling of the A6 as a m7. In the reverse, a dominant or secondary dominant can assume the role of a German sixth chord by respelling the m7 as an A6. This device allows for modulations to distant keys.

In the example below, the V⁷ chord in C minor becomes the Ger⁶ chord in B minor.

D. Enharmonic reinterpretation of a Fr⁶ chord
into a root position french chord
in a major/minor key a tritone away

A less common modulatory device is to consider a root position French chord in the home key as enharmonically equivalent to a French sixth chord in a key a tritone away or vice versa. For example, C to F and f:

In the example below, the Fr⁷ chord in C major becomes the Fr⁶ chord in F minor.

Other Types of Modulation

A. Phrase Modulation

In a phrase modulation the composer ends a phrase in one key and then abruptly starts the next phrase in a new key.

B. Common-tone Modulation

In a common-tone modulation the composer uses a single note, often the ending melody note, as a pickup note to move from one key to another. The most typical common-tone modulation is to use the root of the tonic chord (for example, G in G major) as the 3^rd of a chord in the new key, or vice versa. This approach allows for a modulation to keys a M3 or m3 either up or down: III, III, VI, or VI.

Summary of Modulations