Wrote a tune, and learned about tritone substitution.
When my jazz class ended at noon, I sat down at the classroom’s piano and started messing around. By the time I had to leave for my 2pm class, I had come up with the framework for a tune that starts in the key of B♭ and ends up in the key of B. When I got back to my dorm at 3pm, I sat down at the piano in the basement and played for another three-and-a-half hours, adding more to the tune and trying to figure out how to shift the key back to B♭ at the end.
In the middle of that, I looked up something online about jazz piano and saw someone mention tritone substitutions, which I was not familiar with. I messed with that for a little bit and found that it’s possible to use a tritone substitution to modulate down a half step. Here’s an example starting in B.
BM7 BMm7 FMm7/C | B♭M7 B♭Mm7 EMm7/B | AM7 etc.
So I was able to apply this new discovery when writing this tune: Since it started in B♭ and ended up in B, I could use this trick to modulate it back down to B♭.
Here are the chords:
||: B♭ E♭m7/G♭ | B♭ E♭m7/G♭ | B♭ E♭m7/G♭ | E♭m7 A♭/E♭ :||
B♭ E♭m7/G♭ | B♭ E♭m7/G♭ | B♭ E♭m7/G♭ | B/F# BMm7/F
||: EM7 DM7 | D♭m7 CM7 | Bsus4 Bsus4/C | Bsus4/D♭ Bsus4/D :||
B F/C | B F/C | B F/C | FMm7/E♭ FMm7 | (repeat tune)
An interesting thing to note is that both the A♭ at the end of the first line and the B at the end of the second line act as IV chords. The A♭ acts as the IV of E♭, and the B acts as the IV of F#/G♭ (the major triad of which is contained within an E♭m7, and is emphasized when voicing E♭m7 with a G♭ on the bottom).