Vizualitation of all chord progressions! (kinda)

4

C → G → Am is the same as A → E → Fm

If you mean F♯m, and both of them are encoded with the same Roman numerals in the relative major, then yes. So if C - G - Am is in C Major but A - E - F♯m is in A Lydian, the former yields I - V - vi but the latter is IV - I - ii in E Major. The Trends API is still not quite complete and I would like to see more progress on this after the API is more usable.

The numbers are as they are represented in the Trends search string, here in EBNF metasyntax:

(* Roman numerals *)
           numeral = "1" | "2" | "3" | "4" | "5" | "6" | "7";

(* Borrowed modes, from Dorian to Locrian *)
              mode = "D" | "Y" | "L" | "M" | "b" | "C";

(* Figured bass for triadic and seventh chords *)
         inversion = "6" | "64" | "7" | "65" | "43" | "42";

(* Functions available for applied chords *)
          function = "4" | "5" | "7";

(* Basic chords or borrowed chords in the relative Major key *)
      simple-chord = [mode], numeral, [inversion];

(* Applied chords *)
     applied-chord = function, [inversion], "/", numeral;

(* Chord progressions for both the Trends page and the API *)
             chord = simple-chord | applied-chord;
trends-progression = chord, {".", chord};
   api-progression = chord, {",", chord};

This is reverse-engineered and does not include added/suspended chords or supermodal chords such as “4add9” or “S(3)3”, which occasionally show up in Magic Chord. “add9” and “sus4” probably go after [inversion], “S(3)” probably belongs to mode as do other supermodes. All progressions generated can be parsed unambiguously.

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