I am not a jazz musician. I prefer to stick to rock or folk, and I see a lot of what modern jazz musicians do as showing off or needless complication of what should be simple. And so, for a long time, I resisted learning many ideas and techniques that I thought of as "jazz techniques," or "jazz theory," or even "jazz chords."
I was, of course, being silly. There is no such thing as a "jazz technique," just as there is no such thing as a "jazz chord." Jazz musicians (most of them, anyway) are trying to do the same thing the rest of us are trying to do: make music that sounds good. So, while you may not want to bust out the augmented seventh chords in your next Woody Guthrie cover, a lot of the tips and tricks that jazz musicians use can be applied in other contexts. One of these tricks is the concept of "chord substitutions."
Wait, don't run away!
Chord substitutions sound scary, because we hear people talk about things like "ah yes, the quintessential tritone substitution with the dominant seventh over a flat fifth blah blah blah." It really doesn't have to be that way. A substitution is just replacing one thing with another thing. In this case, it's replacing one chord with another chord.
You can really substitute any chord for any other chord, but just plugging in chords willy nilly doesn't work so well. For one thing, it can end up sounding like a completely different song. For another thing, your band-mates will get mad at you.
What goes in for what?
There are usually at least two parts to any song performance: the melody, meaning the singer or the soloist; and the harmony, meaning the long-haired dude strumming the guitar and making faces. Chord substitution is for the harmony dude (or dudette). For now, let's assume the melody is going to stay the same.
So say the original song calls for a C chord, with a certain melody over it. You're bored with C chords, so you want to plug something else in, but you don't want the song to sound like a train wreck. How can you pick a good substitute?
Keep as many notes the same as possible
There's your answer: pick a chord that has a lot of the same notes as the original chord. That still gives a lot of options, so let's keep things simple, and limit ourselves to chords that are in the song's original key (let's say key of C for this example), and limit ourselves to three-note chords.
Here is a table listing all the three-note chords in the key of C major, laid out so you can compare which chords have similar notes. The notes in the C chord are highlighted
This chart makes it really obvious that the Em and Am chords each share two notes with the C chord. The Em has E and G, while the Am has C and E. You can feel comfortable substituting either of these chords for the C major, and the melody will still usually sound good.
This is as simple a substitution scenario as you can get. When you get into chords with four or more notes, the possibilities increase exponentially. I will address that in a future lesson. For now, let's analyze these results.
As usual, we can turn the chord names into Roman numerals to get a general rule. In this case, the ii chord and the vi chord can substitute for the I chord in a major key. This applies no matter what key we're in. Plugging in the vi for the I is very common in both improvising and songwriting. Remember the sixth chord in the major scale is the relative minor, so it has a very close relationship. (I discussed the concept of relative minor previously.)
Some substitutions are better than others
Looking at these results, the initial impression might be that the Am and the Em are equally good chords to substitute for the C. However, that is not really the case. For one thing, when replacing the C chord, it is nice to have a C note in the new chord, so the Am (A, C, E) is better than the Em (E, G, B). The other reason is that B note in the Em chord. If the melody has a C note in it (which is likely since it called for a C chord), you get a minor-second interval (a.k.a., half-step) between the B and the C. This can often be an unpleasant sounding interval. (If you don't believe me, try playing the second string, fourth fret along with the open first string on your guitar. Bleah!) For these reasons, Am is often a better choice than Em here.
You can even play the Am as Am/C. (Read about slash chords.) This keeps the C note in the bass line and makes the substituted chord even closer to the original. Even looking at the chord shapes, you can see how similar they are:
C major chord
Doing it again
Here's the same chart, but highlighting the notes in the G chord instead of the C.
Here we can see that Em and Bdim each share two notes with G. Em is the relative minor of G, so that shouldn't surprise us. The Bdim you probably won't use much, since we're not really big into diminished chords around here. Feel free to give it a try, though. If you like the sound, go for it.
I encourage you to try this exercise with some of the other chords. There are certain rules of thumb, like the relative-minor one, that pop up over and over. I'll summarize these in a future lesson. And of course, remember that the subs can go both ways. That is, if you can plug in an Am for a C, you can plug in a C for an Am, and so on.
When to do it
There are two main occasions when chord substitution comes in handy: improvisation and composing. When jamming with some friends and you're sick of playing the same four chords over and over, why not try a substitution for one or more of them? The worst that can happen is the song sounds a little off for a measure or two. But you can also discover a brilliant new sound and keep your interest more focused.
Likewise, when composing a song, chord substitution can really come in handy. For example, in one of my songs, I have a chorus that follows the chord progression A, Em, A, D a couple times, then I switch it up for one iteration to A, G, A, Bm. I just plugged in a G for the Em (the relative major) and plugged in an Bm for the D (the relative minor). Then I go back to the original progression to finish up. It gives the song a nice sense of variety and built up tension.
It's also a great technique when stealing -- I mean taking inspiration from -- other songs. Taking a cliched chord progression and changing out one or two chords can lead you to something new and surprisingly different.