The_Complete_Idiot%27s_Guide_To_Music_Theory

Other Extensions
While the seventh chord is almost as common as an unadorned triad, other
chord extensions are less widely used. That doesn’t mean you don’t need to
bother with them; when used properly, sixths and ninths and other extended
chords can add a lot to a piece of music.
Let’s look, then, at the other extensions you can use to spice up your basic chords.
Sixths
I said previously that all chords are based on notes a third apart from each other.
There’s an important exception to that rule: the sixth chord. With a sixth chord
(sometimes called an added sixth chord), you start with a basic triad; then add an
extra note a second above the fifth—or a sixth above the root. You can have
major sixth and minor sixth chords, as well as sixths above diminished and augmented
triads, as shown in the following figure:
Different types of sixth chords.
Later in this chapter you’ll learn about chord inversions, where the order of the
notes in a chord is changed. Interestingly, a sixth chord can be viewed as nothing
more than the first inversion of a seventh chord.
For example, the C Major 6 chord (C E G A) contains the same notes as the A
minor 7 chord (A C E G), just in a different order. For that reason, you sometimes
might see sixth chords notated as seventh chords with a separate note (the third)
in the bass. (C Major 6 could be notated like this: Am7/C.) This is a little
advanced—come back to this sidebar after you’ve read the section on inversions.
It’ll make sense then.
Ninths
Note
A ninth chord adds another third on top of the four notes in the seventh chord.
That makes for five individual notes; each a third apart. You can have ninth chords
based on both major and minor triads, with both major and minor sevenths.
Here’s just a smattering of the different types of ninth chords you can build:
Different types of ninth chords.
Chapter 9: Chords
Note
When you get up to the
ninth chord, you assume
that the chord includes
both the underlying triad
and the seventh.
119