Blues Harmony and Theory

Let’s take a look at the chords in the blues, and what they imply.

The basic blues revolves around three chords. There are sometimes others, but let’s consider these three as the core of the blues universe.

Basic Blues Chords in C

Roman Numeral

Chord Name

From these chords, we can extrapolate the pitch content of a blues in C as being the sum of all notes found in the chords. Then we can arrange those notes as a scale for use in the blues context.

Following the Chord Tones

Pitch Content in C Blues

Scale constructed from notes in blues chords

This scale has some problems, though. Just because a pitch fits well with one chord in the blues, that doesn’t mean it necessarily fits with the others. For instance, the B that we would play over the G⁷ chord is actually completely inappropriate for the C⁷ chord, because the C⁷ chord has a B♭, and these tones conflict. So knowing the chord tones helps, but doesn’t complete the story.

For each of the blues chords, there is a scale of pitches that fits. This scale is the Mixolydian scale.

C⁷ Chord Scale (Mixolydian)

F⁷ Chord Scale (Mixolydian)

G⁷ Chord Scale (Mixolydian)

Comparing the resulting chord scales

Now we can see that some tones show up in all three Mixolydian scales, and therefore fit well with the blues. We can create a new scale to use with the blues this way. Let’s look at which tones fit best, based on how many times they appear in the above scales. (Once, twice, or all three times.)

Ranking the Tones

From this we could conclude that the best scale to use with the blues is the major pentatonic built on the IV chord, or the F major pentatonic (F, G, A, C, D) as all of those notes are in every scale. However, the note F itself is not ideal to play over the tonic chord (C⁷). Even though it’s in the scale associated with the tonic chord, it’s what some people call an “avoid note” because it is a half step above a third of the chord and sounds very out. Let’s rule that out.

Additionally, even though E♭ is not in the tonic chord, it sounds good because it’s not a half step above any of the chord tones in C. It also sounds okay over the G chord as well, even though it’s a half step above the fifth note of that chord. Additionally, the tone E sounds pretty good over all of the chords, except the F⁷. And finally, the B♭ sounds pretty good over all of the chords, too. It’s in the I⁷ and the IV⁷ and it’s not in the V⁷ but it’s not one of the “avoid notes” that is a half step above a chord tone.

What we’ll notice is that the blues structure tends to give us a bit more tolerance for notes that may not fit perfectly. If we played some of these sounds outside of the blues, they would sound off. But in the blues it sounds natural.

Most improvisers will use either a C major pentatonic (C, D, E, G, A, C) or a C minor pentatonic (C, D, E♭, G, A, B♭) over the C blues. However, if they use the C minor pentatonic, they will often bend the E♭ up to E, especially over C⁷ tonic chord. And in addition they will sometimes encounter licks that use the other pitches we’ve talked about. But actually, you can use both C major and C minor pentatonic scales over the same blues.

Let’s combine these two pentatonic scales together, and call them something silly, like Master Blues Scale.

Master Blues Scale (Pentatonic Major + Pentatonic Minor)

When dealing with a blues situation, you can generally draw on the Master Blues Scale as a primary source of pitches. Then you can add color notes to the blues, which are generally context dependent. Color notes are any notes that we discussed already, but which are not in the Master Blues Scale.

Repeating the ♭3 to ♮3 Effect

In many cases, once a musical idea is proven sound, that opens the door for other “twists”—re-uses of the same idea in another context. The ♭3 to ♮3 of the blues (E♭ to E in our example) is one case of this.

Because the Master Blues Scale is very heavily defined by the sound of a ♭3 of the I⁷ resolving up to the ♮3 of the I⁷, this relationship can also be used on the other chords. In the case of the C blues, the F⁷ has “A” as its ♮3. This note is sometimes preceded by A♭, emulating the relationship first established on the tonic chord. That introduces a new note, A♭, into the blues. The same can happen for the V⁷ chord, in this case between B♭ and B, but both notes were already in the blues, so no new pitch is added, and it’s a little less special.

The So-Called "Blue Note"

We still haven't even talked about the famous "blue note". This term is most often used to describe another color note that has these properties:

• The blue note is between 4^th and 5^th of the tonic chord. (F♯ in our example)
• The blue note is almost always approached from the 4^th below. (F in our example)
• The blue note is almost always a very short duration, not long enough to sound dissonant. (a grace note)
• The blue note almost always resolves up to the 5^th of the key, or back down to the 4^th it came from.

However, using the term "blue note" to refer only to this note is a bit of a misnomer. Actually, many notes that we've talked about can be thought of as blue notes.

Color Notes and Their Uses