The_Complete_Idiot%27s_Guide_To_Music_Theory

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Part 5: Embellishing
Chord extensions can make a basic chord sound lush and exotic. There’s nothing
like a minor seventh or major ninth chord to create a really full, harmonically
complex sound.
Seventh chords—especially dominant seventh chords—are common in all types
of music today. Sixths, ninths, and other extended chords are used frequently in
modern jazz music—and in movie and television soundtracks that go for a jazzy
feel. Pick up just about any jazz record from the 1950s on, and you’ll hear lots
of extended chords. There are even a lot of rock and pop musicians—Steely
Dan comes to mind—who embrace these jazz harmonies in their music.
So why not use this technique yourself?
Note
Seventh chords have been part of the musical vocabulary from about the seventeenth
century. There is a tendency to use the V7 and ii7 chords as much as or
more than the triads on those degrees of the scale—even for the simplest musical
genres, such as hymns and folk songs. In the blues, it is common to use seventh
chords on every scale degree—even the tonic.
Other extended chords (ninths, elevenths, and so forth) came into widespread use
in the nineteenth century, and are still used in many forms of music today. (Chopin is
often cited as one of the first composers to extensively use extended chords.) For
example, in many jazz compositions the ninth, eleventh, and thirteenth chords are
used more often than triads and seventh chords.
Here’s an example of how extended chords can make a simple chord progression
sound more harmonically complex. All you have to do is take the standard
I-vi-IV-V progression in the key of C (C-Am-F-G) and add diatonic sevenths
to each triad. That produces the following progression: CM7-Am7-FM7-G7—
two major sevenths, a minor seventh, and a dominant seventh. When you play
this progression—and invert some of the chords to create a few close voicings—
you get a completely different sound out of that old workhorse progression.
And it wasn’t hard to do at all!
The standard I-vi-IV-V progression (in C) embellished with seventh chords (and some close
voicings).
You can get the same effect by adding ninths and elevenths to your chords
while staying within the song’s underlying key. The more notes you add to your
chords, the more complex your harmonies—and the fuller the sound.