Q: When is it ok to use chord substitutions on standards? Also, can you explain tritone substitutions?
A: The use of chord substitutions is fun to practice but must be used with caution on the gig. Our role as a bass player is to support the harmonic foundation for the rest of the band, as well as lock the rhythms with the drummer. Bassists should see themselves as a bridge between the drummer and the pianist or guitarist. When we start trying to change the harmony – or imply a different harmony – we will affect how the rest of the band plays and sounds.
If the rest of the band knows how to listen well and react, it can be a beautiful thing. But there is always the possibility that you will just throw people off, or sound like you got lost or forgot the changes.
This means that we have to play close attention to context.
If you don’t play a lot of standard jazz gigs, you might want to stick to the tune, for the most part. But always listen so if someone else wants to take it in a new direction, you’ll be able to happily follow along and trying to either compliment or contrast the new direction in a musical way.
The easiest example of a chord substitution a bassist might employ is that of a first inversion (3rd of the chord in the bass). This is pretty safe, as your still playing in a consonant way, not rubbing the harmony in a harsh way.
Generally speaking, I think chord substitutions are best left to the chordal instruments but the second part of your question relates well in as much as you can often use a tritone substitution for a dominant chord, often causing a half-step resolution as opposed to resolving by a 5th.
A tri-tone resolution is basically replacing a dominant chord with another dominant chord, the root of which is a tri-tone away (sharp 4 or, literally, 3 whole-steps away (ergo TRI-tone).
This works because of the function of a dominant chord and how it resolves to a tonic. If we look at a G7 chord, we have G B D F.
The F and B notes happen to be a tri-tone and have a bit of dissonance in the sound but both resolve by a half step to the CMajor chord tones of C and E by a half-step. In other words, the 3rd of the G7 resolves to the Root of CMajor and the F resolves to the 3rd of CMajor.
Now, if we replace G7 with a C#7 (the root of which is a tri-tone away) we get C# F G# B. While the C# and G# create a bit of a rub, the F and B still resolve nicely to the new tonic, creating an equally well functioning dominant chord as our original G7 chord!
So, if we had a progression like this:
D-7 | G7 | C∆7
We could reharmonize the progression as:
D-7 | Db7 | C∆7
Note: I used Db instead of C# because of the downward motion. C#7 is the same as Db7.
Using just those simple examples of 1st inversion and tri-tone subs can be a nice sounding an effective way for a bassist to alter the changes they are playing without throwing off the rest of the band.