(See below NB for a simple guide to musical notes and tones)
In music, depending on what notes you use in a single chord, you can produce beautiful harmonies as the tones complement each other. The opposite of this is called dissonance and it results in a harsh, unpleasant sound. A famous example of this is a tritone – a chord made from two notes exactly three whole tones apart. In a standard C major diatonic scale (which doesn’t involve any flats or sharps), there is only one tritone per octave: F and B. But on the chromatic scale (all keys), any number of tritones are possible.
Historically, the tritone has been the black sheep of music theory due to its dissonance crashing any harmony of a song and being difficult to sing. It sticks out like a sore thumb among the sea of beautiful harmonies that other tones make. The tritone was hated so much so that it was named diabolus in musica (“the devil in music”) or the devil’s interval since the Middle Ages, even being banned in the production of music prior to the Renaissance. To this day, the tritone is suggested as an “evil”, “scary” sound.
Over time, composers worked around the tritone until they realised that thanks to the connotations, the tritone was a useful way to express “evil” in a musical way. The cultural association was exploited freely in works such as Franz Liszt’s Dante Sonata, where the tritone is used to depict Hell. The association is found in modern music as well to produce an unsettling feeling, such as the opening notes of Jimi Hendrix’s Purple Haze. The tritone is a common feature of heavy metal bands such as Black Sabbath.
Even though these songs use the devil’s interval, they are not at all inferior to “normal” major scale music. They are still beautiful in their own, interesting way. Perhaps the notion of good and evil have no place in judging whether something is beautiful or not.
NB: Musical tones are noted using the alphabet: C, D, E, F, G, A and B, with a flat(b) to denote a semitone lower, or a sharp(#) to denote a semitone higher. This is easy to visualise on a piano keyboard, where a single tone interval involves a white key, a black key in between and another white key. The interval between a white key and a black key is a semitone.
(Image: Portion of Hieronymus Bosch’s The Garden of Earthly Delights depicting musicians’ hell)