The Complete Idiot''s Guide to Music Theory

Part 3:
Tunes
112
Forming a Chord
Okay, here’s the formal definition: A chord is a combination of three or more
notes played together.
Let’s do a little exercise: Sit down at the nearest piano and put your right thumb
on one of the white keys. (It doesn’t matter which one.) Now skip a key and
put another finger on the third key. Skip another key and put a third finger on
the fifth key. You should now be pressing three keys, with an empty key between
each finger. Press down and listen to the music—you’re playing a chord!
Basic chords consist of just three notes, arranged in thirds, called a 
triad.
The
most common triads are constructed from notes plucked from the underlying
scale, each note two steps above the previous note. So, for example, if you want
to base a chord on the tonic of a scale, you’d use the first, third, and fifth notes
of the scale. (Using the C Major scale, these notes would be C, E, and G.) If
you want to base a chord on the second degree of a scale, use the second,
fourth, and sixth notes of the scale. (Still using the C Major scale, these notes
would be D, F, and A.)
Building a three-note triad.
Within a specific chord, the first note is called the 
root
—even if the chord isn’t
formed from the root of the scale. The other notes of the chord are named rel-
ative to the first note, typically being the third and the fifth above the chord’s
root. (For example, if C is the chord’s root, E is called the third and G is called
the fifth.) This is sometimes notated 1-3-5.
Different Types of Chords
Let’s go back to the piano. Putting your fingers on every other white note, form
a chord starting on middle C. (Your fingers should be on the keys C, E, and G.)
Nice sounding chord, isn’t it? Now move your fingers one key to the right, so
that you’re starting on D. (Your fingers should now be on the keys D, F, and A.)
This chord sounds different—kind of sad, compared to the happier C chord.
You’ve just demonstrated the difference between major and minor chords. The
first chord you played was a major chord: C Major. The second chord was a
minor chord—D minor. As with major and minor scales, major and minor
chords sound different to the listener, because the intervals in the chords are
slightly different.
The notes of a chord don’t
always have to be played
in unison. You can play
the notes one at a time,
starting (usually, but not
always) with the bottom
note. This is called 
arpeg-
giating
the chord, and the
result is an 
arpeggio
.
Note
You should always
spell a triad using every
other letter. So D
-F-A
is a
correct spelling (for a D
Major chord), but the en-
harmonic spelling of C
-F-A
is wrong
Warning

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Chapter 9:
Chords
113
In most cases, the type of chord is determined by the middle note—the third.
When the interval between the first note and the second note is a major third—
two whole steps—you have a major chord. When the interval between the first
note and the second note is a minor third—three half steps—you have a minor
chord.
It’s no more complex than that. If you change the middle note, you change the
chord from major to minor.
Read on to learn all about major and minor chords—as well as some other types
of chords that aren’t quite major and aren’t quite minor.
Major Chords
A major chord consists of a root, a major third, and a perfect fifth. For example,
the C Major chord includes the notes C, E, and G. The E is a major third
above the C; the G is a perfect fifth above the C.
Here’s a quick look at how to build major chords on every note of the scale:
Major triads.
There are many different ways to indicate a major chord in your music, as
shown in the following table:
Notation for Major Chords