If you picture the devil, he’s holding a trident; but if his pitchfork is a tuning fork, it is sure to produce a tritone. It’s a trying tone, the tritone, and not a typically trite one; most people would rather try not to intone it. Even Triton, the merman messenger of the deep, who blew a terrible sound on a conch, would not fancy it (it’s more of a dry tone) – and that other Merman, Ethel, would have been no less cagey about it. The only ones producing the interval would be the sirens. I don’t mean the famous singing sisters, either: I mean the two-note alerts on emergency vehicles in places such as England. Walk over to your piano (or someone else’s) and play B and F. You will likely find it neither good nor rich.
But why is it a tritone? Is it really tri plus tone? Yes, it is. It is a difference of three whole tones (six semitones). In the C major scale, the fourth (i.e., the fourth note counting upwards, with C as the first) is F, and the fifth is G. Now, if you start at F instead of C, the fifth is C – a fifth and a fourth make up an octave. But F# (F-sharp) is the tritone of C and vice-versa: there are twelve semitones in an octave, so six is halfway, and two tritones make an octave. So why doesn’t it produce a nice harmony? Because the frequencies increase logarithmically: each semitone is 1.06 times the frequency of the previous, and so the frequency 50% higher – which does make a nice harmony – is actually seven semitones up, the fifth, for example from C to G. And the tritone is so not it. It is a diminished fifth – or an augmented fourth.
The tritone has a mythos such that if you ask a musician about the it, they will be sure to mention directly that it’s the “diabolus in musica,” the devil in music. Stories abound about its being anathematized in medieval times, singers who dare voice it risking excommunication, but these are mere stories; the dissonance was disliked for obvious enough reasons, but the term “diabolus in musica” is only attested starting in the eighteenth century. And by the middle of the twentieth century, this archetypal dissonance was being used thematically in classical music – it’s central to Benjamin Britten’s War Requiem, for instance.
But the tritone is also a central note to “the devil’s music” – blues, which were, incidentally, the original basis for heavy metal music (which, however, has moved quite some ways away from blues in the intervening decades). In the hexatonic scale of the blues, it is the pivotal note, and it is called the “blue note.” Which is not to say it runs afoul of blue laws. But be wary of it if liquor is restricted – you don’t want to be caught with a diminished fifth.