Why does Amaj Cmaj Dmaj Amaj work so well?
How can we describe the relationships between these chords?
What key am I even in? Amaj and Dmaj could be I and IV in A major, or Cmaj and Dmaj could be IV and V in G major? Feels like the former, but how do we explain the Cmaj in An A major key, borrowed parallel?
Is this modal or something?
Any suggestions as to where/how you could develop this progression further?
You could well be in A major. Does A major feel like home? ♭III is a frequent visitor to a major key.
Try adding another common interloper, ♭VII. So you'll have Amaj Cmaj Dmaj Gmaj Amaj. Nice?
Non-diatonic chords don't necessarily change the basic tonality. And chord progressions don't HAVE to fit a 'circle of 5ths'.
A C major in the middle of an A major context can work as a "modal interchange" or "borrowing" or temporary key change to parallel minor Am or something. It's a common thing in jazz and blues, flirting with and alternating between parallel major and minor tonalities.
