Principle 21.3.2. Augmented Sixth Chords.
When you encounter a chord with a diminished third, it is an augmented sixth chord.
Section 21.3 Analyzing Augmented Sixth Chords
Since our process for analyzing chords has been to stack them up in thirds to determine the root and quality, it is worth examing these four augmented sixth chords as stacks of thirds.
Notice that all augmented sixth chords contain a \(\left.\text{}^{\circ}{}\right.\)3 when stacked in 3rds. None of the chords we have studied so far contain diminished thirds.
Once you determine the chord you are analyzing has a \(\left.\text{}^{\circ}{}\right.\)3, analyze the scale degrees to determine which type of augmented sixth chord it is.
