Harmonizing Scales

Let’s use the C major scale to keep things simple.

C D E F G A B C

To create a C major chord off this scale, we take the root, third and fifth: C E G. Now let’s move everything up one degree to the second, forth and sixth, D F A, which makes a D minor chord. Continue moving up the scale in this fashion and you get:

C Dm Em F G Am Bdim

I ii iii IV V vi vii

We then give these chords numbers based on what degree of the scale the root is. These are the common triad chords in the key of C major. Now let’s turn them into a chord progression, by moving along the cycle of fourths, meaning we move up in fourths, which will give us each chord once and only once before coming back to C.

C F Bdim Em Am Dm G C

I IV vii iii vi ii V I

This by definition is the strongest chord progression you can form, the chord switches also get stronger as you move towards the right, V-I is the most common progression, and is so strong that when we hear the V chord our ear WANTS to hear the I chord, to the extent that going to any other chord will sound harsh.

Now obviously playing one chord progression all the time will get boring, so here are some simple guidelines to add variation.

I is the strongest chord, so it can go to any other chord and still sound good.

iii and vi can sub for I

ii and IV can interchange

V and vii can interchange

Remember that ALL rules have exceptions, and we’ll examine them later.

The Explanation:

The reason for these substitutes actually comes from the four note chords rather then the triads, let’s examine I, vi and iii

A
C   C
E   E   E
G   G   G
   B   B
       D

Both chords share three notes in common with the chord they can sub for.