Hum the first two notes of “The Simpsons” theme song. (If you’re not a Simpsons fan, “Maria” from West Side Story will also do.) The musical interval you’re hearing—the pitch gap between the notes—is known as a “tritone,” and it’s commonly recognized in music theory as one of the most dissonant intervals, so much so that composers and theorists in the 18th century dubbed itdiabolus in musica (“devil in music”).
Now hum the first few notes of Twinkle Twinkle Little Star, or, if you prefer something with a little more street cred, the “I’m sorry” part in Outkast’s “Ms. Jackson.” This is the “perfect fifth.” It’s one of the most consonant intervals, used in myriad compositions as a vehicle of resolution and harmony.
Read the whole story: Scientific American