Major Sixth

Nine semitones, a ratio of five to three, which is simple enough that the interval sounds consonant despite being wide. Invert a minor third and you get a major sixth, so the two are the same pair of notes seen from opposite ends.

Open and optimistic. My Bonnie Lies Over the Ocean opens on a rising major sixth, and being a traditional melody it is safe to use as a reference.

Heard as a leap it is easy. Heard inside a chord it is harder, because it blends in rather than standing out.

In sixth chords, where it replaces the seventh and gives a finished sound. In minor sixth chords, where it is the bright note in a dark chord and the mark of Dorian. In pentatonic melodies, where the leap from the fifth to the sixth turns up constantly.

With the minor seventh, one semitone wider, especially inside a chord. And with the minor sixth below it, which is a completely different mood.