The_Complete_Idiot%27s_Guide_To_Music_Theory

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Part 3: Tunes
Chord Extensions
Chords can include more than three notes. When you get above the basic triad,
the other notes you add to a chord are called extensions.
Chord extensions are typically added in thirds; so the first type of extended chord
is called a seventh chord because the seventh is a third above the fifth. Next up
would be the ninth chord, which adds a third above the seventh … and so on.
Chord extensions are nice to know, but you can simplify most pieces of music
to work with just the basic triads. The extended notes add more color or flavor
to the sound, kind of like a musical seasoning. Like a good meal, what’s important
is what’s underneath—and you can always do without the seasoning.
So if you see a piece of music with lots of seventh and ninth chords, don’t panic—
you can probably play the music without the extensions and still have things
sound okay. Of course, for the full experience, you want to play the extended
chords as written. But remember, the basic harmonic structure comes from the
base triads; not from the extensions.
That said, it helps to have a full understanding of extended chords, just as a
good chef must have a full understanding of all the different seasonings at his or
her disposal. That means you need to know how to build extended chords—so
you can throw them into the mix when necessary.
Sevenths
The seventh chord is the most common chord extension—in fact, it’s so common
that some music theorists categorize it as a basic chord type, not as an
extension. In any case, you need to be as familiar with seventh chords as you are
with triads. They’re that important.
Creating a seventh chord within a specific key or scale is normally as simple as
adding another third on top of the fifth of the base triad. This gives you a 1-3-
5-7 structure—the equivalent of playing every other note in the scale.
There are actually three basic types of seventh chords: major, minor, and dominant.
Major and minor seventh chords are kind of sweet sounding; the dominant
seventh chord has its own internal tension.
Dominant Sevenths
The dominant seventh chord—sometimes just called the “seventh” chord, with
no other designation—takes a major triad and adds a minor seventh on top. In
other words, it’s a major chord with a lowered seventh; the chord itself consists
of a root, major third, perfect fifth, and minor seventh. This is sometimes
notated 1-3-5-♭7.
For example, a C7 chord includes the notes C, E, G, and B♭.
The dominant seventh chord is an especially important—and frequently used—
extension, as this is what you get if you play a seventh chord based on the fifth
(dominant) tone of a major scale. As you’ll learn in Chapter 10, the dominant