To answer that, you have to first answer what the dual means in the first place. And the notion that you get to is dual with respect to the octave. A usual cho

To answer that, you have to first answer what the dual means in the first place. And the notion that you get to is dual with respect to the octave. A usual chord is with respect to a fifth. Then a major third is a minor third down. Starting position should be at the tonic. Otherwise, you get at the fifth that when you go a major third down it is a minor third from the tonic.