# D to F in key of C: Secondary Dominant, or something else?

In the Cee Lo Green song “Forget you”, the chord progression is C - D - F - C, the D has an F# which is out of key.

Within the limited music theory I learned so far, the closest thing I can relate this to is secondary dominant? Like C - D - G - C, the second D major chord is out of key and if you read it literally, it’s a D major chord, but really it functions like V/V, five of the five chord, which is G, which resolves to the next G chord, then back to C.

Is this C - D - F - C progression similar to C - D - G - C?

(UPDATE)

After reading answers and other resources I don't think the D is functioning as a secondary dominant in this particular case

• 'Eight Days a Week' - Beatles, same sequence, except no 7th.
– Tim
Mar 5, 2020 at 9:35
• Thought Experiment: try playing that F chord as F/G (play the G in the bass under an F chord). It'll sound similar, and I think that could be the beginning of an answer... Mar 5, 2020 at 17:14

IF `C` is the tonic, then you probably want to consider @AlbrechtHugli's point in comments.

If you omit the `D7` you get something very, very familiar `C - F - C`. You could call that a kind plagal progression, basically it just shifts back and forth from tonic to subdominant.

Functionally that doesn't go anywhere. It just prolongs the tonic.

Interposing the non-functional `D7` in that prolongation doesn't really change what is going on: an elaboration of a tonic.

The root movement by step from `C` to `D` creates a nice linear movement but that isn't the same as actual harmonic function.

Back to @AlbrechtHugli's point, some call this a Lydian progression because of the raised fourth degree. That particular wording Lydian progression suggests a kind of borrowing of chords from the Lydian mode. Borrowing like that is more about color (chromaticism beyond the diatonic) without those chromatic tones having actual function, the critical one being chromatic tones which become temporary leading tones.

If you want to label it with Roman numerals, you can use a capital letter for the major quality and add a seven: `C: I II7 IV I`

You're correct, it is a secondary dominant or V/V chord. It does function similarly to the Dm or ii chord, except that the non-diatonic F# tone provides a stronger leading tone to the G than the diatonic F natural in the Dm chord.

However, it should be pointed out that although that is the V/V chord, it does not resolve to the dominant, but instead to the subdominant. So I suppose the argument could be made that it serves more of a generic predominant borrowed chord function. And the plagal cadence from F to C differs from the ordinary G-C cadence also. At any rate, those are the differences between this progression and C-D-G-C.

• If it's functioning as a secondary there should be something following that makes sense with a `G` tonic, `G` major or minor. Mar 5, 2020 at 17:44
• @MichaelCurtis I've heard the argument that secondary dominants don't have to resolve to their respective tonics before - In C major, E7 sometimes goes to F, and I see that get called a secondary dominant resolving deceptively all the time. Mar 5, 2020 at 18:13
• "resolve to their respective tonics" right, that's why I didn't say that. The point is for something to happen in `G`. Mar 5, 2020 at 18:35
• Your `E7` sort of makes that point, `E7` would suggest an `A` minor tonic, going to `F` actually would be a deceptive progression in `A` minor. It depends on the exact example. If the progression just continues with clear cut harmony in `C` like `|:C E7 F G:|`, I don't like calling that a secondary function. Why? Because it's waffling around with the function of `F`. Is it a subdominant or not? Taking a basic functional progression `I V` and the interrupting it with another purported functional move of `V7 vi` seems like a really weak analysis of function. Mar 5, 2020 at 18:40
• What to label that `E7` doesn't seem like a big deal provided the critical `I ? IV V` is properly identified. If you really want to call it `C: I V7/vi IV V` with a secondary to nowhere, or just label it as a chromatic chord `C: I III7 IV V`. Mar 5, 2020 at 18:43