Here’s a musical concept nobody taught me at school.
While analyzing 1000+ jazz tunes for The Jazz Standards Progressions Book, I noticed a great number of songs in major keys tonicize the IV degree at the beginning of the bridge.
It’s usually preceded by the V/IV, landing on the IV at the beginning of the bridge, then finding it’s way back to I towards the end of the bridge, usually via the V/V, the IIm7-V7 or a similar harmonic path. Here’s a good example:
Had there been a second cadence in the bridge using a secondary dominant resolving towards the IV, then I would have considered it a modulation at the beginning of the bridge. However, many times the music “tonizices” the IV for just a few measures, then it goes back to other harmonic functions in the main key. That is why I talk about “tonicizing” the IV degree instead of modulating to it.
When I was choosing the chord scales for each harmonic function I realized that the default chord scale for a IVmaj7 chord (Lydian) does not quiet work. I find that an Ionian chord scale just sounds better in this case even though we have not modulated. I did a lot of listening and I hear the great improvisers play out of an Ionian chord scale in this context. I think the reason for this scale choice is this “quasi modulation” which calls for an Ionian sound while harmonically the tonal shift isn’t strong enough to consider it a modulation.
That’s what granted us to create a new chord/chord-scale pairing option for the IVmaj7 and the IV6 in Mapping Tonal Harmony Pro adding the Ionian scale as an alternate choice. We feel it represents what’s going on musically more faithfully than using a Lydian scale and better than making a brief modulation to the IV, then quickly modulating back to the original key. There are no grounds to justify such a short modulation to a key that’s so closely related.
Mapping Tonal Harmony Pro turned out to be the ideal tool for this project. It allowed us to transcribe each song as we analyzed it and save it to a catalog, all in one step. Then we used that catalog to produce each volume of The Jazz Standards Progressions Book.
Here’s the complete tune with full harmonic analysis, chord-scales and arrows & brackets analysis. Take the A Train: