The tritone – that interval of 3 whole tones (augmented fourth/diminished fifth) – harsh, dissonant, one of the cornerstones of our music since about 1600 because of its tension and then demand for a sweet release.
Those two notes (F-B) put together were prohibited for centuries, especially in any church setting. Around the time Guido of Arezzo was developing our modern staff notation (over 1000 years ago already), it was called a “dangerous” interval, Diabolus in Musica (the devil in music). Even today, one of its uses is to suggest an oppressive, evil, or scary atmosphere.
Back in the Medieval and Renaissance times, musicians took steps to avoid this. One of my favourite examples is a little song from the 14th century called Puis Qu’en Oubli (Machaut). At the eleventh second there is a chord so unexpected that it sounds wrong because we are okay with the tritone these days, and, in fact, we expect it here. It happens again at 0:21 before “resolving” to an open fifth – another unexpected and slightly disconcerting sound to our ears. I’ve never had a student not love this in the end – it opens their ears to a different harmonic system – almost jarring in its surprise.
*In the piano-centrism and ignorance of my youth, I had the audacity to call anything written before Bach “pre-music.” I remember being stunned when I met a recorder major at University – I had no idea one could major in an grade 3 school instrument. The Early Music crowd descended on the UBC campus every summer, providing proof that they are alive and well and the music is flourishing, and that I was just wrong. Thank you, Heather, for reminding me of all this :)