[laGrenouille]

Interesting quantitative take studying chord distribution. The basic takeaway seems to be that if you re-phrase a song into a different key you can play a lot with just a few chords (I IV V), and even more with a few others.

While I would agree that you'll be okay playing any major scale in C major (or whatever other major key choose to learn), playing a song in a minor scale on a major scale just doesn't sound quite right. So, I'd double all of the numbers on their final table to account for learning a full set of major and minor chords.

[circlefavshape]

If you know I, ii, iii, IV, V, vi in C then you'll be able to play tunes in A natural minor. Add III to cover A harmonic minor, and II to cover A melodic minor

In real life though the key usually depends on the singer's vocal range, so you won't really get away with learning just one key

[tarentel]

In A harmonic minor the III is already a C, you can make it augmented but that is not very common. You generally change the v to a V and keep the VII diminished in both cases where as it is major in the natural minor.

[laGrenouille]

Yes, that's a good point in the case of six keys (I think my point holds for just learning three), but it is not the one made that the article seemed to make.

[LtWorf_]

But A minor is the same notes as C major.

The chords in the key of C major will be the same chords in the key of A minor, although their patterns might be different.

[tarentel]

This is only partially true. The chords in A natural minor will be the same but it's much more common to hear a dominant fifth using the harmonic minor so you would change the Em to E especially in classical and jazz music.