Remember this line from that last post?
You can substitute a half diminished chord for a minor chord of the same spelling if it is functioning as a ii chord for a target that is also minor.
That is one confusing bit of writing so let me do my best to clarify it for you today. Let’s start with a basic concept: a target chord.
A target chord is simply a destination chord in a song. Some targets are sort of obvious such as the chords at the ends of phrases. However, if you are arranging and you look at a song, pretty much any chord you see can be a target chord. Furthermore, your target chords do not have to be the same as my target chords. The targets you pick are largely a stylistic decision rather than a right or wrong one.
Another way to look at a target is simply as a chord that ends a progression. Think of your targets as anchor chords that you will not change because they define the harmonic character of the arrangement. However, the other chords that you stick in front of those targets are negotiable. The only rules are that the chord progressions you use need to be functionally defensible and they need to work with the melody notes.
Let’s take a very simple example. Here is the basic chord progression for the first line of “Just As I Am.” Each chord represents one bar.
I I V I
It is sort of obvious that the last I chord is a potential target and I am going to treat it that way. It is set in stone. However, what chords can go in front of that last chord? It depends; there are numerous possibilities. I am going to essentially wipe out those chords (except the first chord of the song) and start over with a clean slate.
There are lots of ways I can go. For example, here is an option based on the circle of fifths. The chords that I am sticking in front of the target flow in a logical and functional way.
I iii – vi ii – V7 I
The truth is I could throw a few hundred other examples down here but that is not the point. I do want to give one other example though that looks very similar:
I iii – VI7 ii – V7 I
Those of you that are theory buffs know what I did. I changed the minor vi to a VI7 which technically turned it into a secondary dominant. It becomes a V/ii. And when I do that, I am essentially creating a new target: the ii chord.
I ii – V7 I
The ii chord is now a chord that I am inserting chords in front of. In this case, iii – VI7 technically are functioning as a ii – V of the ii. We would write this as ii/ii – V/ii.
I ii/ii – V/ii ii – V7 I
Now, once again, I could throw a lot of different chord progressions in front of that target ii. I could use iii – biiidim7 for example. However, by far, the most common progression that you stick in front of any target chord is the ii – V (of the target chord). In other words, if you are in the key of C and you choose a target Am chord, the two chords you would be most likely to stick in front of it are Bm7 – E7. While those chords could be labeled as vii – III7, they really are functioning as ii/vi – V/vi.
That brings me back to the rule. If your target is a minor chord and you are sticking a ii – V in front of the target, it is a pretty safe bet to substitute half diminished for minor on the ii. Use the same spelling. (In other words, if Dm7, use Dm7(b5).)
In the example I just gave in the last paragraph, turn that Bm7 into Bm7(b5). Or, if you are targeting Dm, play Em7(b5) – A7 in front. And by the way, if you are playing in the key of C minor rather than C major, you would do the same thing when moving to the I chord. In other words, you would play Dm7(b5) – G7.
Got it? If not, here is one more way of explaining it…
If you identify a target chord in your song that is a minor chord
AND you want to insert the relative ii – V in front of it….
…Convert the ii from a minor 7th to a half diminished chord.
Why does this work? There is a logical reason that has to do with leading tones but I will not go into it here. I will say this though. For the very same reason this works, I would also encourage you to add a flat 9th to the V (which happens to be the same note as the flat 5th of the ii). In other words…
Dm7(b5) – G7(b9) – Cm
Try it. You will like it.