I’ve heard stories about this, but I think it may be hogwash. Did the Church believe that a certain chord was the Devil in music, or did people just know that it doesn’t work harmonically or melodically? Was anyone ever punished for using the chord?
Yes, that refers to the tritone (augmented 4th or dimished 5th). en.wikipedia.org/wiki/Tritone
I doubt people actually thought the devil was in it. But they clearly thought it was somehow a “damaged” interval. One could call it a fallen interval, I suppose.
I have no idea whether anybody was ever punished for using it. I highly doubt it.
The tritone (i.e. the augmented 4th/flat 5th “diabolus in musica”) is inherently dissonant - either monophonically (as an interval) or homophonically (as a “chord”).
Here’s the scoop: When Gregorian chant is sung homophonically, it’s sung with the harmonic voice a 4th or 5th separated from the melodic voice. The tritone is in between the 4th and the 5th (augmented 4th/flat 5th). When harmonizers hit their notes badly out of pitch (a half-step is pretty wide), they can slide into the tritone… and thus into dissonance. Apparently, this did happen at times in the monasteries. Some people just can’t sing.
If I took a chant and harmonized it using a tritone rather than a 4th/5th, you would immediately hear the dissonance.
Frankly, I don’t know if the label “devil in music” was originally intended in earnest (i.e. the monks authentically saw the hand of the devil messing with their harmonies) or if it was suggsted somewhat wryly (as with a bit of Irish wit, for example). But I do think that ultimately, people did think of it as a truly diabolic interval. I would imagine that by the late Medieval period, anybody who intentionally harmonized using the tritone would have been subject to some scrutiny.