Regardless of the genre, how would it be best to describe the chord progression (the song is in C major) of C-A-G-F…the body of the song then goes to an alternating D-G. So, we have (in the key of C) I-VI-V-IV then II-V in the body of the song. I believe we are looking at secondary dominants; if so, is C the V F?; is A the V of D?, etc., etc. Thank you. John
Great question here!
The progression that you outline is in the key of C major, and we know that the V7 chord in the key of C Major is G7.
When we see D- and G, here written as triads, this implies the 25 progression which resolves to the tonic C Major. The G7 would not be classed as a secondary dominant chord, because G7 is the diatonic V7 chord in the key of the tune/progression - C Major.
So what is a secondary dominant chord?
A secondary dominant chord is when we approach a non-tonic chord in your progression with its V7 chord.
For example, the second chord in your progression is A- which is the vi- chord in the key of the C major. The V7 chord of A- would be an E7, so we could play the following progression:
C / E7 / A- / G / F —> back to C to start the progression again
In this example, the E7 is a secondary dominant chord.
Here is a short video to show you some ways that we can incorporate this into the progression:
You will notice in my progression that I made a few other substitutions:
Instead of G I played a G9sus - this gives a more modern flavour, also it sounds unresolved like it wants to move somewhere which keeps the progression moving forwards.
I substituted the F triad for a D-7 (the F triad exists within the D-7 chord and so these chords are very similar and can often be interchanged). I believe I played a D-9 for extra colour.
We are then moving back to C to start the progression again, and so I played a G7 (or G13b9 to be exact) which creates a 251 back to C major (D-9 to G13b9 to Cmaj).
The G7 is a regular dominant chord as it resolves to the tonic.
The E7#5#9 is a secondary dominant because it is the V7 of a chord in the progression which is not the tonic. That’s the important point to understand.
Any questions here let me know.
…now I understand what secondary dominant is. Very good video !