Those of you who are trained musicians will be quite familiar with the tritone, or diminished fifth. It occurs naturally as a pitch seven-fifths above a fundamental. Placed above a major third to the fundamental in the root position of a chord (inversion notwithstanding) called a dominant seventh, it is part of the most familiar cadence in post-Renaissance Western music - the perfect cadence.
Removed from an aesthetically pleasing position and simply played with the fundamental and - for good measure - an octave, the tritone is famously and aptly known as "Diablo in Musica." It tends to conjure all sorts of nasty, Heronymous Bosch-like visions of the devil and his minions crawling out of the upright piano, on which you would most likely be incessantly playing this interval, to wickedly and perversely "reward you."
Now, fast forward four hundred years. Have you ever noticed that computer printers, when in operation, almost unanimously emit tritone noises?
What were those Luddites talking about, again?
(As an aside, I'm bothered as to how quickly my academic knowledge of music has slipped in three years. I took 21 credits in college, finished both the music theory and ear training courses. I know the stuff intuitively - but writing this, I'd need to go back to my theory books to score a German, French or Italian augmented sixth. Italian's missing the third, right? Can't recall from the top of my head. Damn disuse!)
UPDATE: I took a quick refresher on augmented sixths. Cancel alarm. Everything's cool.