How would you know if a song is in a minor key or major key, just by looking at the chords? The chords of C major and A minor are identical right? So how would I know what key the song falls under? That would also bring up the question of why a relative minor needs to exist? Like a 2-5-1 in E minor would be the same as a 7-3-6 progession in G major, so why? HELP!?