The tritone is an interval so dissonant that it has earned the nickname ‘The Devil’s Interval’, and it was avoided for centuries by composers and the pupils they taught. Centuries ago, using the tritone went against everything music appeared to stand for, and it was named “diabolus in musica” – “the Devil in music”.
Why is the tritone perceived as dissonant?
Pitches sound harmonious together when there is a simple ratio between the frequencies of the two notes. The ratio of two notes an octave apart is a pleasing 2:1, and these sounds are so consonant we even give both the notes the same name. The next most consonant interval is the perfect fifth, (3:2). And so it continues, through the full range of intervals.
So how about our friend the tritone?
Its notes end up in a ratio of the square root of two in our modern tuning system, which is called equal temperament. This ratio sounds pretty ugly to the human ear. Rumors abound that music featuring the tritone was banned from churches because of its association with the devil. But as juicy as this gossip about the tritone might be, there is no evidence that this ever happened.
It’s more likely that the strict rules of harmony observed by those composing for the church forbade the use of the tritone for musical reasons, rather than because they believed that it was the devil dwelling in music.