I would like to ask you to provide a theoretical explanation for the fact that fourths and fifths are not classified as major-minor. It cannot be because they are the same in both major and minor scales, for the second is also the same and it is not considered a perfect interval. If it is pointed out that minor seconds also occur in the scales, I would reply that a diminished fifth also occurs. Why wouldn’t that provide the basis for classifying fifths in major and minor? Or is it only because of medieval usage of these intervals as perfect? Thanks a lot.
I think it has more to do with tradition than it does with any specific sound. The perfect intervals tend to have the smallest ratios between notes:
- Unison – 1:1
- Octave – 2:1
- Perfect 5th – 3:2
- Perfect 4th – 4:3
But the major and minor third are not far off:
- Major 3rd – 5:4
- Minor 3rd – 6:5
Keep in mind too, that with equal tempered tuning these are not the actual ratios used. But these were the tunings used by the greeks, and early Christian churches, where our theory began to cement itself.
For more – read https://en.wikipedia.org/wiki/Interval_(music).