Speak low - tonal mapping


I am a beginner organ player.

If you look at the chord progression for the attached excerpt of ‘Speak Low’, it goes like this :

| Gm7|C7 |Gm7
C7| Gm7|C7 |Am7(b5)
Eb7|Am7 D7|Gm7|C7

I think this starts as a 2-5-1 in the key of F, but then it pivots elsewhere, using the Am7(b5) chord, but based on the chords that follow, I dont recognize the progressions or the key. I am guessing 2-5-1 minor in the key of G, but there are still some chords I am wondering about…Bbm7 and Eb7

Can anybody explain this harmonic trajectory?

Thanks in advance !

Hi @angelo, thanks for writing!

Answer to your question:

First, the Am7b5 D7 is just a II V to G-7, so still within the key of Fmaj, even if it doesn’t resolve to G-7.

The Bb-7 Eb7 25 can be explained in many ways, here 2 ways I’d like to think of it:

Bb- in the key of Fmaj is so called IV minor, “borrowed” from F- key, and can be used as a part of a 25 Bb-7 Eb7

Another way to think of it is:

Eb7 is one of the 4 dominants that can resolve to Fmaj. 4 dominants are: C7, Eb7, Gb7 and A7.

This concept is explained thoroughly on the following course:

Let me know if you have any further questions,