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Altered common chord modulation is common chord modulation in which the annotated common chord is one of the common types of altered chords (like Neapolitan chord or Augmented sixth chord) either in the old key, new key or both. Such a common chord is called an altered common chord.
Example of altered common chord modulation
Example of altered common chord modulation
The next figure shows the measures 51 to 53 from Chopin’s Prelude No. 17 in Ab major. We see two brief modulations: first from the E major key to the Eb major key and then from the Eb major key to the D major key. These two modulations are between distant keys.
The diatonic tones of the E major key and the Eb major key are also shown in the figure. We see that they differ in all the note names. Thus, to find the exit point, we must search for the last occurrence of any diatonic tone of the E major key. To find the entry point, we must search for the first occurrence of any diatonic tone of the Eb major key. The exit point is at the end of measure 51 and the entry point is at the beginning of measure 52. The corresponding notes are marked in blue in the figure.
In this modulation, there is a single altered common chord: the E major triad chord. It can be explained as the tonic triad chord in the E major key and as the Neapolitan chord in the Eb major key. Thus, this is the altered common chord modulation. In fact, the Neapolitan chord in the Eb major key is the Fb major triad chord, since the Neapolitan chord is defined as the major triad chord on the lowered second scale degree. This is the reason why this modulation is also an example of enharmonic modulation. We see that the Neapolitan chord is resolved to a chord with the root on the dominant, which is a standard usage of the Neapolitan chord.
Similar considerations also hold for the second modulation.
Since these two modulations form a modulating sequence, they are examples of the sequential modulation. The harmonic sequence is annotated in the figure.
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