Half-Diminished Blues

Weekly Newsletter #35

April 7, 2022

When you play a standard 12-bar blues song what chords do you typically use? 

For the average player, the dominant 7 or dominant 9  seems to be the chord of choice. For example, in an A blues something like an A7 or A9 would likely be what most players gravitate towards. 

But what if you want to do something more than just play that one A7 chord? 

That’s where basic chord substitution can come in handy. Learning how to “sub” chords in place of a dominant 7 can range from something easily done to a complex multi-layered process. 

To get you going and playing some cooler and more colorful chords over a standard blues, we’ll start with the fastest and easiest substitution there is…the Half-Diminished chord. 

What is a Half-Diminished Chord? 

Known commonly by it’s alternate name, m7b5, the half-diminished chord is most regularly found as the seventh chord of any key. 

An example would be in the key of C you could build a four-note chord (1-3-5-7) upon each note of the scale: Cmaj7 - Dm7 - Em7 - Fmaj7 - G7 - Am7 - BØ 

To truly understand what we’ll be discussing, it’s vital that you’re familiar with the concept of chord scales and how seventh chords are created. For an in-depth explanation on these please read Newsletter #24. 

The last chord of that C major scale is the BØ. That symbol “Ø” means half-diminished but is also commonly written as Bm7b5 (B minor 7 flat 5). Keep in mind the BØ = Bm7b5, they are the exact same chord, just written using different symbols. 

If we break down what this chord is comprised of, you should get a better understanding of how it can be used. 

The half-diminished chord is essentially a “diminished” version of a minor 7 chord, hence the m7b5 name. 

If you took the four notes of a Bm7: B-D-F#-A (1-b3-5-b7) and you shrunk the 5th by a half-step you’d end up with a Bm7b5: B-D-F-A (1-b3-b5-b7). 

This is the case with all half-diminished chords, they are just one note removed from their parallel minor 7 counterpart. 

Many intermediate players understand the major scale chords and use almost all of them. However, the last chord of the major scale is often the one a lot of players don’t know what to do with. That is about to change for you. 

I Still Don’t Know What To Do With This Chord! 

Now, just because there is only one note difference does not mean that this half-diminished chord will sound good in place of a Bm7; in fact it’ll probably sound terrible. 

This is because the consonant and “solid” structure of the perfect fifth that exists in the Bm7 has been removed. In its place is a flat 5th (b5), which creates an extremely dissonant sound between the root of the chord and that b5. 

To test this out, simply play a B and F# together, and then play a B and F together. 

Notice how the B and F sound really tense and almost scary? That’s the dissonance I’m talking about, and it’s why it will likely sound pretty bad if you use it in place of the Bm7. 

However, that dissonance is the exact reason we’ll be using it in our blues playing. 

Remember I mentioned that we would be substituting a m7b5 in place of a standard blues chord like a dominant 7? Well that is the perfect way to get used to playing this often-overlooked chord. 

Let’s stick to the Bm7b5 and see how we can use it. But first have a look at all the various inversions of this chord. 

How Does This Chord Substitute For A Dominant 7? 

If we were to take the dominant 7 chord that occurs naturally in the key of C we would be playing a G7 chord. Now this G7 can also be expressed as a G9, in which there is one extra note. If we compare the two chords we will see some overlap: 

G7: G-B-D-F (1-3-5-b7) 

G9: G-B-D-F-A (1-3-5-b7-9) 

Knowing that both a G7 and G9 function as dominant 7 chords helps us think of these two as interchangeable versions of the same chord idea. They both serve the purpose of a G dominant sound, only with the G9 having one extra color note. 

Notice that within the G9 lie the exact same four notes of Bm7b5? This is not a coincidence. 

It is why when you play the C major chord scale and you go from the BØ back to the C it sounds like a resolution. The same way play G7 or G9 to a C major chord sounds like a resolution. 

This is because it is a resolution. 

The important notes of a G9 chord are all found as part of the BØ. 

That’s Great, But How Do I Use This? 

The first thing is understanding that Bm7b5 is a direct substitute for G9 because all the notes of Bm7b5 are found within a G9 chord. Take a look at the notation to better understand this: 

When you play that G9 chord shape, you are playing the Bm7b5 within it. Just remove the root note G and voila! You have a BØ! 

Another easy way to think about this is that when you’re playing a normal 6-string bar chord version of a G7 all you have to do is go back one fret and up one string to B on the 5th string 2nd fret. From there you can make the most common shape of the Bm7b5 and use that as a direct replacement for G7. 

This simplistic way of visualizing the substitution of the Bm7b5 for the G7 can easily be transferred from G7 to any other dominant 7 chord. 

Want a replacement for an A7? Simply think of an A7 bar chord and go back one fret and up one string to the C# and play the half-diminished shape. 

Learning all four inversions of these half-diminished chords on each of the bass strings will be a huge help in getting you comfortable with actually playing them in real life. 

Now to put this into practice means you have to be able to quickly find the substitution chord for any dominant 7. The easiest way to do this is simply to build the half-diminished chord on the 3rd of any dominant 7. 

The 3rd of G7 is B = Bm7b5. 

The 3rd of C7 is E = Em7b5. 

The 3rd of D7 is F# = F#m7b5. 

Putting this into practical use may take some getting used to, but here is an outline of a way to incorporate these chords into your blues playing. 

Below you can find an AUDIO version of what this example sounds like. 

The limits to this are endless, but getting comfortable with not only substituting these half-diminished chords in place of a dominant 7, but being able to jump freely between the inversions and use them melodically and rhythmically will take your blues playing to a much higher level. 

-Max Rich