Hi Bill. I saw a video about Disclosure's song "Latch" where it said the song was in Bb dorian and went i > v > IV > III. Someone in the comments said no, it's clearly in Ab major and goes ii > vi > V > IV. Someone else said no, it's in F minor and goes iv > i > VII > VI. It was all a bit silly and it made me wonder if I should just analyze all songs in a major key even if they're minor or another mode. E.g. it seems a lot easier to just remember that vi > IV sounds cool rather than remembering both vi > IV and i > VI since they are the same notes. Do you see any downsides in only thinking about chords from an "ionian perspective"? (this might be a cool math rock band name) Or are there benefits in calling the vi chord the i chord if the song is in a minor key?
if it's an ambiguous case, i generally will gravitate toward the 'ionian perspective' as you call it. i would surely consider Latch in Ab, especially when you look at the melody. Some songs may be much more strongly anchored to that 2 root, where you would definitely wanna say 2 dorian. like miles davis "so what", no one is going to say that's in C. or "uptown funk", no one's gonna say that's in C. it clearly lives its life in D dorian, period. There are many many ambiguous cases, especially when you can't decide whehter to take the "ionian perspective" or the relative minor ("aeolian perspective", as you might say). Like a song that is firmly in A minor in the verse, then does F C G G for the chorus, obviously the chorus is in C. Or the ever so popular perpetually looping Amin F C G sequence, that's the most famously ambiguous case. The whole thing sounds like it's in C except that the first chord is actually Amin. Some songs are ambiguous as to which way you'd call it, but the melody is really what holds the answers