Music is a powerful thing: It can raise our spirits, carry us through athletic challenges, and make us weep. Its very fabric is a source of power and intrigue, too, since just a measly few tones might do anything from shatter glass to manifest the Devil’s Tritone.

The Devil's Interval, and diabolus in musica, this combination of tones has led to some of the most chilling melodies in music history, from classical compositions to heavy metal riffs, and even has a reputation for being banned by religious authority in centuries past. As much as it’s inspired composers to explore the dark side in music, however, the Devil’s Tritone—a.k.a. the diminished fifth—also has a stirring effect on audiences for some very technical reasons (no black magic required).

THE RESTLESS, DISSONANT, DEPENDENT TRIAD (SAY WHAT?)

For those of us without conservatory backgrounds, a break-down of the musical terms used to define the Devil’s Tritone can go a long way in helping unravel its eerie mystery.

According to Carl E. Gardner’s 1912 text Essentials of Music Theory, a “triad” in music is composed of three tones—specifically, one starting note plus the third and fifth tones found along its scale (e.g. C, E, G)—that can get together to form either a “dependent” or an “independent” chord. According to Gardner, an independent chord is one that can happily conclude a composition. Meanwhile, a dependent chord contains “dissonant” or tense intervals—such as the tritone.

One example of a dependent chord containing a tritone would be the diminished chord (e.g. C, Eb, Gb). According to Gardner, a chord containing this kind of dissonance is "restless" and shouldn’t be given the last word in a composition lest the audience—and any traditional music theorists among them—are left feeling uncomfortable.

WHAT GIVES THE TRITONE ITS POWER?

Music listeners’ almost instinctive desire to hear a song through to its rhythmic and harmonic conclusion can be an effective (if torturous) tool throughout the fields of music composition and scoring. The last moments of The Sopranos’ series finale are likely extra-irksome to many, for example, not just because of unresolved plot points, but also the unresolved chorus in Journey’s “Don’t Stop Believing” is left hanging when the screen has gone black.

John Sloboda, a professor of music psychology at London's Guildhall School of Music and Drama, explained to NPR in 2012 that the dissonant intervals of the Devil’s Tritone are particularly affecting because of this listener’s instinct to find resolution in music, and the fact that we’re used to getting it:

"Our brains are wired to pick up the music that we expect, [and] generally music is consonant rather than dissonant, so we expect a nice chord. So when that chord is not quite what we expect, it gives you a little bit of an emotional frisson, because it's strange and unexpected.

The emotional result of dissonant sounds, then, might not be too different from the one experienced at the bottom of a staircase that failed to mention it’s missing its last step. "[Music] taps into this very primitive system that we have which identifies emotion on the basis of a violation of expectancy," Deathridge said. "It's like a little upset which then gets resolved or made better in the chord that follows."

WHERE CAN I FIND THIS DIABOLUS IN MUSICA?

Some say that the devil’s in the details, and if you listen closely, you’ll indeed spot the Devil’s Tritone giving a certain edge to many popular tunes from different genres. It heats up Busta Rhymes’s “Woo Hah!! Got You All In Check,” the theme songs to The Simpsons and South Park, and West Side Story’s “Maria.” It also gives Jimi Hendrix’s “Purple Haze” its signature sting. It’s a favorite among metal bands, too, and can be found in any number of Black Sabbath songs (though guitarist Tony Iommi told BBC News that he simply used “something that sounded right … really evil and very doomy,” and that he “didn’t think [he] was going to make it Devil music”). Prog-rocking Rush even manages to shred its way through both ascending and descending tritones multiple times in its four-and-a-half minute, decidedly epic song "YYZ."

But the Devil's Tritone's deepest roots are in classical music, where it has often served as a leitmotif to signal the presence of something sinister. Professor John Deathridge of King's College London told BBC News that medieval arrangements employed the tritone to represent the devil, Roman Catholic composers sometimes used it for referencing the crucifixion, and by the 19th century "you have got lots of presentations of evil built around the tritone" in classical pieces, as in Beethoven’s 1805 opera Fidelio. When it comes to metal’s cred with tritones, there’s “a big connection between heavy rock music and Wagner,” Deathridge said, and generally such tunes “have cribbed quite a lot from 19th Century music."

Overall, the Devil’s Tritone “can sound very spooky [depending] on how you orchestrate [and] also quite exciting," Deathridge said. "[Wagner's] Gotterdammerung has one of the most exciting scenes—a 'pagan,' evil scene, the drums and the timpani. It is absolutely terrifying … like a black mass.” Musicologist Anthony Pryer pointed out that the leitmotif lives on as an arrow toward evil on-screen, too: "[a] lot of films have what musicians call Captain Tritone in them,” he told BBC News, or moments wherein an enemy officer or such shows up and “out comes the Tritone [as] a sort of badge—here's Mr. Nasty. What's going to happen?"

WAS THE DEVIL’S TRITONE REALLY BANNED BY THE CHURCH?

Over the years, there have been rumors that the diminished fifth tritone was banned by religious authorities, or even that composers were punished for sneaking it into their work. Given that various Christian faiths and organizations have either produced or influenced much of the classical Western canon, though, experts seem to think it’s more likely that musical monks and other religious composers discouraged its use in keeping with “strict musical rules,” Deathridge said. “This particular dissonance … simply won't work technically, [so] you are taught not to write that interval. But you [could] read into that a theological ban in the guise of a technical ban." Pryer notes, too, that the tritone “was recognized to be a problem in music right back to the 9th Century [and] a natural consequence, and so they banned it [and] had rules for getting around it ... I don't think they ever thought of it as the Devil dwelling in music.”

According to Pryer, there are a number of non-accursed ways this tritone could’ve gotten its name. “It was called Diabolus in Musica by two or three writers in the medieval or renaissance [because it] was 'false music,'” he explained, since “the intervals weren't natural.” On the other hand, composers and conductors may have found it “devilishly hard to teach the singers not to sing it,” he said.

In the case of Giuseppe Tartini’s “The Devil's Trill Sonata”—one of the absolute toughest pieces a violin virtuoso can take on—the mark of the beast might be twofold. According to Pryer, "He did this incredibly difficult [piece] and claimed in a dream he had heard the devil giving him instructions how to do it … Two centuries later, he would probably have been in a heavy metal band."