Around the beginning of the 11th century, a Benedictine monk by the name of Blessed Guido of Arrezzo invented the staff notation – the first organized systematic way of writing and reading music.
Before his staff notation system, ecclesiastical singers would have to spend a great deal of time memorizing music, which frustrated Guido. The system would eliminate the need for memorization, meaning singers could diversify their studies in prayers and other religious texts.
He garnered a reputation for being able to teach large amounts of music quickly, and drew singers from around Italy seeking his instruction.
There is one sound he would not teach his pupils however, a sound he called diabolus in musica, Latin for ‘the Devil in music’: the tritone.
The tritone is a melodic dissonance, evoking such feelings of unease and despair that for hundreds of years a pervasive myth persisted that it’s singers were excommunicated by the Church.
“When we hear something dissonant, it gives you a little bit of an emotional frisson, because it’s strange and unexpected. The emotional result of hearing a tritone, 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.”
Because of its avoidance in medieval ecclesiastical singing and Guido calling it the Devil in music, it became symbolically associated with evil, and wasn’t used by composers until the rise of the Baroque and Classical music era.
The real reason it was called the Devil in music? Besides its dissonant sound, it was technically hard for singers to sing.
It was easier to avoid it in music altogether than have ecclesiastical singers try to incorporate it into Gregorian chants and the like.
Listen below to an example of the tritone: