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Recently I went to a LA Chamber Orchestra talk on the history of Beethoven’s 4th Piano Concerto, a work that is famous for throwing out the rulebook of the concerto form (and also having some bits that sound so much like Chopin, it’s eerie).
The conductor mentioned a term I hadn’t heard before – Diabolus in Musica (The Devil in Music)- a medieval term that was given to the tritone, an interval that was deemed so dangerous, people thought it conjured up the devil. Â It’s the interval of three whole tones dividing an octave, so a C natural to an F#, for example, and it was expressly forbidden in church music according to cannon law.
It’s also an augmented 4th, which is the interval between the first two notes in “Maria” from West Side Story. Â It longs to be resolved. Â Can you imagine the Ma-Ri without the A at the end of it? Â It’s desperate for a resolution. Â So having it just hanging there seems unnatural and sinister.
From an article in Token Rock:
http://www.tokenrock.com/secret_resonance/tritone_interval.php
Some say the tritone represents the Devil because it is a dissonant interval with an irreconcilable split ratio of 7:5 (augmented 4th) or 10:7 (diminished 5th) as found in meantone temperament. But dissonance cannot be the only reason. The tritone is not much (if any) more dissonant sounding than the intervals of a minor 2nd or major 7th and no one thinks they are devilish. They’re not even naughty.
Some say it was the Devil in music because the tritone is so close to the interval of a perfect 5th that two monks could too easily sing dissonantly as they tried to chant in pure parallel 5ths. But this cannot be the only reason because when they sang out of tune anywhere else, those wrong intervals weren’t the Devil. They were just out of tune.
It received a revival in popularity during the 19th century, and the second movement of Beethoven’s 4th Piano Concerto uses it liberally, as it supposedly tells the story of Orpheus going down into Hades to tame the devil. Â Interestingly, a lot of heavy metal bands use it now. Â Slayer actually has an album entitled “Diabolus in Musica,” paying homage to the tritone.
The whole thing really interests me – that just a few hundred years ago people actually thought that the devil could live in music, and be summoned by an interval. Â We think we’re so clever now, but how many of us still freak out over Ouija boards, or tarot cards? Â I wonder whether, in five hundred years, someone will bring up card games, and say, “and even into the 21st century, many people still believed that tarot cards had some type of power to summon the devil or spirits.” Â Our brains are so funny.
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Heather Teysko is the creator, writer, and producer of The Renaissance English History Podcast,  one of the longest running indie history podcasts, going since 2009. She's been writing about history online for over 25 years, since her first site on Colonial American history became number one in history on Yahoo in 1998. She writes books, created the original Tudor Planner (a yearly diary filled with Tudor history) and in 2019 created the world's first Tudorcon (you can "attend" online this year!). She has been passionate about Tudor England since she first read Alison Weir's The Six Wives of Henry VIII 20 years ago, and subsequently moved to London after college to spend her time immersed in Tudor history.