Just what is it about the tritone chord progression that makes it so mysterious? Perhaps it has to do with its dissonant, clashing sound. It may have something to do with its dubious history. Whatever it is, the tritone chord progression in music made a comeback in music after several centuries of bad publicity.
'Tritone" is defined as a musical interval that spans three whole tones. A tritone chord may also be called an augmented fourth or diminished fifth chord. The tritone sounds like a clash, or as a dissonant chord. For this reason, the tritone chord was often avoided during Medieval times through to the end of the Romantic era.
For hundreds of years musical styles were, in large part, dictated by the church. During Medieval times, the tritone was viewed as too dissonant for use in common liturgical services. In fact, the tritone chord progression came to represent the devil. Perhaps as early as the 18th century it was commonly known as "diabolus in musica" (the devil in music).
A great deal of superstition came to be associated with the tritone. Many church fathers adhered to the belief that it may even serve to invoke the power of the devil. Because of this superstition, the use of the tritone was banned by the church for liturgical use. Because of this negative association, even secular music produced during these centuries avoided it.
There is speculation that this chord may have been associated with the Devil for another reason. The tritone, as already mentioned, consists of three whole tones.
Three whole tones equal six semitones. This may have led the church fathers to associate the tritone with the Biblical "mark of the beast," or number of the devil: 666.
As with any widely held superstition, the tritone had a bad public image to overcome. Eventually some musicians cautiously experimented with the tritone, particularly during the Baroque and Classical music era. Finally, it seemed as though its stigma had been somewhat overcome during the Romantic period. Notable classical musicians like Vivaldi, Beethoven and Debussy inserted the tritone into various works.
When the equal temperament system of tuning came into general practice in Western music, the tritone began to make a comeback in contemporary songs. Still, it had remnants of its former reputation. The tritone began to appear in modern rock and roll, jazz and blues songs. Those with prudish natures denounced it, probably still subscribing to the old-fashioned church-propagated superstition. Despite some opposition, the tritone took hold. Today it's used regularly and without inhibition.
Many musicians are still aware of its diabolical history. In fact, the tritone is sometimes still used in contemporary media to signify, represent or "invoke" the devil. One example of this is the 1986 movie Crossroads. In it, the main character, in a showdown of guitar prowess, ends a guitar solo with a tritone chord because of its association with the devil. However, its relation to ancient superstitions has been largely forgotten by the general public. Today, the tritone is used artistically, just another color in the musical palette.