Lesson 34 Introduction to augmented sixth chords
Part I - Augmented sixth chords
So far, all of the chords you have studied have been built in intervals of thirds. There is, however one more chord in functional harmony that is built on an interval other than a third. This is the augmented sixth chord. This chord consists of either three or four notes, depending on its configuration. each configuration has a unique name based on the inner voices of the chord. each chord will always take the lowered sixth (le) as its root. The other note that will always be in an augmented sixth chord is the raised fourth (fi). This interval between the lowered sixth and the raised fourth is an augmented sixth (6+)Thus the name of the type of chord.
In a major key, le and fi are both altered notes. Therefore these chords will require accidentals in major keys. However, in minor keys only fi will require an accidental.
Being that the augmented sixth chord is strongly identified with certain scale degrees, it has a very specific function within harmony. The tendency of a lowered tone is to continue to resolve downward. As is the tendency of a raised tone to continue to resolve upward. The two outer notes of an augmented sixth represent both of these tendencies. In figure 1, we see just the outer two notes of the augmented sixth.
These notes will resolve outward to an octave, usually the V chord. Figure 2 shows the resolution of the chord.
You can see that the outer two notes resolve to an octave on G, the V of C Major. It then resolves as one would expect to I.
Part II - Identifying Augmented sixth chords
Since augmented sixth chords are not constructed in thirds as all other chords are, they cause problems when trying to identify them.
The first step in any chordal analysis is to spell the chord. the second step is to see where the chord resolves to. With most progressions, ii, I (or i) six-four, or IV (or iv) would resolve to V. another common problem when identifying augmented sixth chords is that they can often be stacked in thirds as a normal triad. But notice that in the example on the left, the chord has a diminished third between the "root" and the third. where actually, the third of the chord is the root.
the augmented third chord in the progression comes before V
I iii vi ii A6 V I
I iii vi ii A6 I(six-four) V I
In a major key, le and fi are both altered notes. Therefore these chords will require accidentals in major keys. However, in minor keys only fi will require an accidental.
Being that the augmented sixth chord is strongly identified with certain scale degrees, it has a very specific function within harmony. The tendency of a lowered tone is to continue to resolve downward. As is the tendency of a raised tone to continue to resolve upward. The two outer notes of an augmented sixth represent both of these tendencies. In figure 1, we see just the outer two notes of the augmented sixth.
These notes will resolve outward to an octave, usually the V chord. Figure 2 shows the resolution of the chord.
You can see that the outer two notes resolve to an octave on G, the V of C Major. It then resolves as one would expect to I.
Part II - Identifying Augmented sixth chords
Since augmented sixth chords are not constructed in thirds as all other chords are, they cause problems when trying to identify them.
The first step in any chordal analysis is to spell the chord. the second step is to see where the chord resolves to. With most progressions, ii, I (or i) six-four, or IV (or iv) would resolve to V. another common problem when identifying augmented sixth chords is that they can often be stacked in thirds as a normal triad. But notice that in the example on the left, the chord has a diminished third between the "root" and the third. where actually, the third of the chord is the root.
the augmented third chord in the progression comes before V
I iii vi ii A6 V I
I iii vi ii A6 I(six-four) V I
