Can someone describe the theory behind this progression? F#m, C#m, D, A, F#, C#, D, A

Question

Can someone describe the theory behind this progression? F#m, C#m, D, A, F#, C#, D, A

It feels like it resolves on A. I assume it's in key of A, but my theory is weak and forgotten. I normally loop it, don't have it going anywhere else...

I made a few more notes on it… If it is in key of A

vi   F#min

iii   C#min

iv   Dmaj

i    Amaj

F#maj   ?not sure why this fits

C#maj   ?not sure why this fits

iv  Dmaj

i Amaj

Answer 1

When we speak of music theory, we tend to mean "common practice era" theory — the theory of Mozart, Beethoven, and their peers up until the twentieth century. However, jazz and popular music, while they do often adhere to the same theoretical ideas, they also tend to depart from it, and this would seem one of those times. Jazz and popular styles frequently privilege root movement with less concern to the preservation of key within individual chord progressions.

With that in mind, here are two theoretical models to describe it, rather than the theory. Note that both models fundamentally rely on the idea of repetition as establishing a basis for the aural understanding of music, as opposed to a key-based understanding.

Theory 1

The vi-iii-iv-I is fairly straightforward in establishing the key of A. But it also establishes a motion between F# and C#. With that (root) movement established, the shift to major chords rather than minor ones comes across as a color shift, as opposed to a key shift.

Theory 2

The chord progression oscillates between two tonics: C# and A. In more standard theory, this kind of modulation by thirds is commonplace.

F#m - C#m = Plagal cadence in C# (minor)
D   - A   = Plagal cadence in A (major)
F#  - C#  = Plagal cadence in C# (major)
D   - A   = Plagal cadence in A (major)

Answer 2

I view this as a modification of the progression from "Puff the Magic Dragon."

The progression from that song, I–iii–IV–I, would be A–C♯m–D–A in the key of A. The only difference between this progression and your progression is that you replace the opening A chord (I) with F♯m (vi). And this is a very common substitution: theorists since at least the 1800s have viewed this vi chord as a variant of the I chord, claiming that the two have very similar functions. (This is in large part because they share two pitches: here, they both have A and C♯.) As a common example of this substitution, consider the deceptive cadence, where V moves not to I but to vi instead.

So this progression is basically the "Puff" progression but with a substitution in that first chord from A to F♯m.