The following is the explanation I use in lessons to teach the concept of “mirror intervals” to my students:
“At the piano, put both thumbs on middle C. Play D with the right hand, while holding the thumb down. Play B with the left hand, while holding the other thumb down. Both Cs will be pressed, but also D (right hand) and B (left hand). Compare the distance going up from C and down from C.
You are now playing 2nds.
Then, with thumbs still on middle C, now play E with the RH and A with the LH.
You are now playing 3rds.
Continue this pattern until you are playing octaves with both hands, thumbs still on middle C.”
- Compare the distance going up from C and down from C. Count the keys any way that makes sense, but be sure to always count the black keys.
- If the number of keys between the keys you are pressing is different (be sure to count the black keys), the interval is major or minor. Minor is always “smaller”. Major is always “bigger”.
- If the number of keys between the keys you are pressing is the same (be sure to count the black keys), the interval is perfect.
Conclusion: 2nds, 3rds, 6ths and 7ths may be either major or minor, while unisons, 4ths, 5ths and octaves (8ths) are always perfect.
Note: At this time I will not cover augmented or diminished intervals, which has to do with notation and not with sound. For the same reason, I am not yet addressing the subject of “tritones”. I will leave these advanced and difficult concepts for later.