Chromatic Modulation Using Enharmonically Reinterpreted Augmented-Sixth and Dominant-Seventh Chords
We can modulate to distant keys in several ways. Chromatic modulation refers to a modulation to a distant key by way of a chromatic chord. Enharmonic reinterpretation involves respelling and resolving a chromatic chord (usually a German augmented sixth or diminished seventh chord).
The German augmented-sixth chord may be respelled to resemble a dominant seventh chord. If you use the German augmented-sixth from the home key, the resulting modulation will be up a half step. Likewise, you can re- spell a dominant seventh chord to resemble a German augmented-sixth chord. If you use the dominant seventh chord in the home key, the resulting modulation will be down a half step. You can, of course, use this trick with any dominant-seventh chord (say, a secondary dominant), opening up numerous possibilities for modulation.