Exactly because they have effects that sound different and are instances of the same general principle. Edit: The idea is not to stop there. You also have the

Exactly because they have effects that sound different and are instances of the same general principle. Edit: The idea is not to stop there. You also have the minor 7th chord being dual to the major 7th chord (with the octave added implicitly). Those two chords are often used together in jazz music. In this case the interval you use to get the upper note is a 4th. And depending on which interval you are focusing on, and what kind of scale you use (if not chromatic) this simple concept can get much more interesting. I agree that playing for example C E G and then C Eb G sounds like a "change", but I don't think the theory gives you complementary chords in sequence. They connect ideas with each other. In that sense it makes sense for a major chord to be dual to a minor chord.