It would nice in some ways I suppose if I could give you lots of rules for all music situations. That would make it easier; but of course at that point, music would no longer be an art. There are some general rules; but much to the consternation of those who think this is all black and white, even those have changed over time.
As an example of one of those vague areas, I will talk about the 9th. The 9th is a very versatile note to add to essentially any chord to thicken it up and add complexity. It is like first base really; I am not saying your music is going to be revolutionized when you start adding 9ths, but you are at least in the game.
There are two 9ths that you can add: a natural 9th and a flat 9th. The flat 9th is far more interesting than the natural 9th. The natural 9th just soaks into the chord like it has always belonged there. The flat 9th does something extremely powerful to the chord. It does not change the function but it most certainly steps up the character of the sound a ton.
Check out this example for what I mean. This is from last week’s “Lead Me To Calvary.” The circled chord has a Bb which is the flat 9th. Play this line with the Bb and then play it again with B natural (the natural 9th).
I use the flat 9th a ton. Frankly, I use it as much as I can. When I can’t use it, I usually still throw in a natural 9th. But how do I decide which to use? There are a few general rules that guide that.
- The only real chord candidate for a b9th is a dominant chord.
- The b9th sounds most best (from a perspective of voice leading) if the chord that follows that dominant chord is minor. In other words, secondary dominants that resolve to minor chords such as V/ii sound great with b9ths.
- In a major key, even though the V7 does not resolve to a minor chord, it feels right and good to use b9th at least some of the time. (In the example above, that is what I have done.)
- Above all, let your ear be your guide.
In regards to that last point, there were a few places in this arrangement where I intentionally backed off a flat 9th. Here is one of them:
I have a confession to make: I really don’t know why the b9th does not work there. In fact, on some days, it does work for me there but on others, it doesn’t. A flat 9th represents dissonance and dissonance is an acquired taste.
The thing about arranging for others is that this is not all about me. I like dissonance and there are a lot of things I could do to get my dissonance fix. But I have to stand back and predict how you guys will react to a b9th there. My opinion based on doing this a long time is that it would be a bit much for a lot of people in that particular spot. That exact chord in a different spot and context would be just fine with the b9th.
I am not sure if that is helpful but that is sort of how writing works. You start with a set of rules knowing that they are fallible and may change in the next decade and then let your ear make the final decisions. As Duke Ellington famously said “If it sounds good, it is good.”