C and A#

(or C and Bb)

 

These notes constitute an augmented sixth or a diminished seventh interval. They are dissonant, but not displeasing. They help create a musical tension, which like the tritone, begs a harmonic change to a more stable sound. As such, it is a very useful device that appears in most musical compositions. Its graph appears to have a random quality to it.

Click to hear this sound as played on a piano: C & A#