COMMON TONE AUGMENTED SIXTH CHORDS
In chapters related to augmented sixth chords, we saw that they most often appear before a chord with the root on the dominant (the fifth scale degree). However, like in the case of common tone diminished seventh chord when a diminished seventh chord does not resolve in a usual way but embellishes another chord (as was explained in the previous chapter), there exists the case when an augmented sixth chord does the same. Such an augmented sixth chord is called common tone augmented sixth chord. The next figure shows the beginning of the Chopin’s Mazurka Op. 67 No. 4 in A minor. We see the German sixth chord that embellishes the tonic chord. The German sixth chord and the A minor triad chord have two common note names: A and C. When German sixth chord embellishes major triad chord, they have only one common note name. In the figure, we see the annotation for a common tone augmented sixth chord that is used in this book: a horizontal line labeled with “CT” which connects the common tone augmented sixth chord with the chord it embellish.
Figure: Common tone augmented sixth chord in Chopin’s Mazurka Op. 67 No. 4 in A minor
The following figure shows another case from Schubert’s Moments Musicaux Op. 94 No. 6 in Ab major. The key of the composition is Ab major and the key signature is the same. However, the passage is written in the Ab minor key. The German sixth chord and the Ab minor triad chord have two common notes: Ab and Cb. We see that an embellishing chord do not have to appear between two instances of the chord it embellishes. Here, the German sixth chord appears before the single instance of Ab minor triad chord.
