The_Complete_Idiot%27s_Guide_To_Music_Theory

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Part 3: Tunes
Let’s use the key of C as an example, because it’s made up of only the white
keys on a piano. When you play a triad based on C (the tonic of the scale), you
play C E G—a C Major chord. Now move up one key, and play the next
triad—D F A, or D minor. Move up another key, and you play E G B, the E
minor chord. Move up yet another key, and you play F A C—F Major. Keep
moving up the scale and you play G Major, A minor, and B diminished. Then
you’re back on C, and ready to start all over again.
This type of chord building based on the notes of a scale is important, because
we use the position within a scale to describe the individual chords in our
chord progressions. In particular, we use Roman numerals (I through VII) to
describe where each chord falls in the underlying scale. Uppercase Roman
numerals are used for major chords; lowercase Roman numerals are used for
minor chords. To indicate a diminished chord, you use the lowercase Roman
numeral plus a small circle. To indicate an augmented chord, use the uppercase
Roman numeral plus a small plus sign.
Thus, within a major scale, the seven chords are notated as follows:
I ii iii IV V vi vii°
If you remember back to Chapter 2, each degree of the scale has a particular
name—tonic, dominant, and so on. We can assign these names to the different
chords, like this:
I ii iii IV V vi vii°
Tonic Supertonic Mediant Subdominant Dominant Submediant Leading Tone
Of these chords, the primary chords—the ones with the most weight—are the I, IV,
and V. These also are the only major chords in the major scale—and often the
only chords used within a song.
When describing chord progressions, we’ll refer to chords by either their
Roman numerals or their theoretical names (tonic, dominant, and so forth).
You can figure out which specific chords (C Major, D minor, and so forth) to
play, based on the designated key signature.
To make things easier, you can refer to the following table, which lists the
seven scale-based chords for each major key signature.
Scale-Based Chords
Key Signature Chords
C
C♯