Have you ever wondered why some musical intervals are called Major or Minor and others are called Perfect?
When I started studying music, I just memorized that all second, third, sixth, and seventh intervals can be Major or Minor, but the fourth, fifth, and octave can only be perfect. But why?
The Reason
The explanation for this is related to the distance in semitones between the two notes in an interval. Let’s think about the C major scale. The interval C to D is ascending and the distance between the two notes is two semitones, so this is a second interval because we moved two notes in the scale of C (remember that the C Major scale has no sharps or flats notes). If we move in the opposite direction we have C to B which is a descending interval between two notes, so it is a second, but now the distance is only one semitone. So to differentiate the two second interval we call Major second for C to D (bigger distance) and minor second for C to B (smaller distance).
The distance between the notes on the ascending and descending direction give us the understanding why Major, Minor or Perfect interval.
If we do the same analysis for the third interval, we have C to E which distance is 4 semitones and from C to A is 3 semitones, so C and E is a major third and C and A is a minor third. For sixth intervals in the scale of C major, we have C to A with 9 semitones and C to E with 8 semitones, so C and A is a major sixth, and C and E is a minor sixth. For the seventh interval, we have C to B with 11 semitones and C to D with 10 semitones.
Forth, Fifth and Octave
Now, pay attention to what happens to the fourth, fifth and octave intervals. Let’s start with the fourth interval. From C to F the distance between the notes is five semitones in the ascending direction. Now let’s count on the descending direction and we have C to G and in this case, we also have five semitones between both notes.
In the same way, doing the analysis for the fifth interval we have C to G and C to F, both cases with a distance of seven semitones. For the octave, we have twelve semitones on both sides (C to C ascending and C to C descending).
This is the reason why some intervals are called perfect and others Major and minor.
