Was the 'devil's interval' really banned by the Church?

Probably not in the way the legend says. The tritone, an interval of three whole tones like F up to B, really was avoided in medieval church singing, because it sounded so restless and dissonant. Its nickname, diabolus in musica, "the devil in music", is real too. But the famous stories of singers being excommunicated or punished by the Church for singing it are, according to historians, likely fanciful.

The avoidance was musical, not a witch hunt. The monk Guido of Arezzo, who helped create the musical staff, designed his teaching system so singers would step around the F–B tritone altogether. The devilish nickname is harder to pin down. In 1733 Telemann wrote that "the ancients" called it Satan in music, and in 1739 Mattheson said "older singers" called it the devil in music, yet no medieval document using the term has been found. Some scholars still think it was used that early.

What's certain is that the reputation stuck. Liszt used the tritone to suggest Hell in his Dante Sonata; Wagner tuned his timpani a tritone apart for a brooding scene in Siegfried. And Leonard Bernstein built much of West Side Story on it: it is the first interval of the opening Prologue and returns in "Maria" and "Cool".