When part-writing augmented sixth chords, like the assignment on page 276, having a set of things to focus on, and an order to approach the problem, can be a big help. Here’s my approach for part-writing augmented sixth chords:
- If you’ve been given Roman numerals with inversion symbols, then the bass line is given. Write the complete bass line first.
- Remember that in minor keys, there is no chromatic alteration needed for the b^6 root of an augmented sixth chord. In major keys, you will have to lower it a chromatic half step.
- Next, deal with the interval of the augmented sixth. You should always deal with dissonant intervals first. They usually must be approached with more care, and resolved in special ways.
- Spell the augmented sixth interval in your head, and then find the voice that gives you the smoothest voice-leading to it.
- Remember that you should avoid augmented melodic intervals.
- Finally, fill in the rest of the chord tones of the aug. sixth chord according to its type. (Italian requires a doubled third above the root, etc.)
- Moving away from the aug. sixth chord,
- resolve the augmented sixth interval first. (This way you don’t try to move some other voice to the same note, and then not resolve the interval properly.)
- Move the remaining voices smoothly to the remaining chord tones.
- With the Italian +6, the voices doubling the tonic (chordal third) will move in contrary motion to each other.
- The German +6 almost always goes to a cadential 6/4 to avoid parallel fifths. The chordal third is a common tone, and the fifth is either common (in minor), or moves up a chromatic half step (b^3 – ^3).
- The French +6 provides a common tone to the fifth of a V (the fourth above the bass holds to become the fifth above the bass), and the chordal third moves down the leading tone.
The overall approach is to focus first on the given (the bass line), then the special dissonance (the augmented sixth interval), and then the remaining chord tones.