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Like the Beast, it goes by many names: Diabolus in musica (devil in music), the devil’s interval, the tritone, the triad and the flatted fifth. As its Latin moniker suggests, it’s an evil sounding combination of notes that’s designed to create a chilling or foreboding atmosphere.

The interval was given a sinister name since listeners originally found it unpleasant and surprising. Before the tritone became a common tool in rock, listeners expected artists to play chords and patterns that were pleasing to the ear. When a tone that wasn’t mellifluous – such as the triad – was inserted into a musical passage, it was unsettling since it didn’t conform to the listeners’ expectations.

In simple terms, a triad is a fifth played one fret down. To play a flatted fifth with a G power chord, place your first finger on the third fret of the low “E” string. That’s the root note, or the “G.” Now, put your third finger two frets up on the fifth fret of the “A” string. That’s the “fifth,” in this case, the “D.” To complete the power chord, place your fourth finger next to your third finger on the next string over, the “D” string. That’s the octave of the root note, another “G.” Strummed together, these three strings create a pleasant sounding chord (G, D, G). But if you take your second finger and play it a fret down as a Db instead of a “D” it creates a flatted fifth. When played after the first “G” or plucked slowly in a “G” (root note), “G” (octave), Db (flatted fifth) progression, it creates a dissonant or ugly tone, especially with distortion.

This simple technique has been used most effectively in heavy metal, and is often credited to Black Sabbath guitarist Tony Iommi who played it in the song “Black Sabbath” from the band’s 1970 self-titled first album. Although Iommi was untrained in music theory, he devised the three-note passage after listening to a piece of classical music he and bassist Geezer Butler enjoyed by Gustav Holst called “Mars, The Bringer of War” from the suite The Planets (written in 1914). The composition included a triad, and when Iommi imitated the sound on guitar he liked the unsettling feeling it created. He experimented with the passage and slowed it down to a crawl. Then he added a trill to the flatted fifth, repeatedly wavering from Db to D and added vibrato to the other notes to emphasize the tension of the music. Many consider “Black Sabbath” the birth of heavy metal.

Iommi used the technique in numerous other songs as well, playing at different speeds and with varying chord combinations, and when the band became successful the tritone became a staple of the genre. Throughout the decades, countless bands including Judas Priest, Metallica, Slayer, Marilyn Manson and Slipknot have all relied on the devil’s interval to add darkness and power to their music. Slayer even named their 1998 album Diabolus in Musica in homage to the technique.

While the tritone was rarely used in rock music before Black Sabbath – with the notable exception of the opening chords of Jimi Hendrix’s “Purple Haze” – many classical composers relied on the flatted fifth many years before Holst, who died in 1934. Beethoven’s 1805 opera Fidelio is flush with triads, as is Richard Wagner’s 1848 composition "Gotterdammerung." In addition, artists who put together spooky movie soundtracks and relied on creepy sounds used tritones years before Black Sabbath wrote their signature tune. Less predictably, Leonard Bernstein used a triad in “Maria” from West Side Story in the simple vocal chorus line “Ma-ri-a” and Astrud Gilberto and Stan Getz integrated the devil’s interval into their hit “The Girl From Ipanema.”

In the ‘70s and ‘80s many prog rockers have been loyal proponents of diabolus in musica. The beginning of Rush’s “YYZ” is all triads, King Crimson use a descending tritone in “Red” and Primus play flatted fifths throughout the South Park theme song. More recently, the construction has been used by modern composers, alternative rockers and even rappers. The first three notes of The Simpsons theme song (by Danny Elfman) is a triad, the Strokes wrote the devil’s chord into the repeated E to Bb guitar riff of “Juice Box” and Busta Rhymes uses a repeated tritone bass line in “Woo Hah!! Got You All in Check.”

There have been rumors that in the Middle Ages composers and singers were forbidden from using flatted fifths because of the dissonant, demonic tone it creates. Although the term diabolus in musica, was, indeed, born in that era since high clergymen found the tone to be the antithesis of godliness, there is no evidence that the technique was ever officially banned. Still, it was so distasteful to the church that no one dared to integrate it into their music.

Today, however, thanks, perhaps to Black Sabbath - flatted fifths are fair game in everything from rap to extreme metal. As for their effectiveness, the devil’s in the details.

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