I often mix up pairs like thirds and sixths, fifths and fourths, seventh and seconds. How can this be explained?
This probably has to do with so-called complementary intervals. If you take a minor third, and raise the lowest note an octave, you get a major sixth. Similarly, a major third is complementary with a minor sixth, a perfect fourth with a perfect fifth, a minor second with a major seventh, and a major second with a minor seventh. It is a logical, very human, but unwanted source of confusion.
