" The tritone - an augmented fourth (or, in hazz parlance, a flatted fifth) - is a difficult interval to sing and has often been regarded as having an ugly, uncanny, or even diabolical quality. Its use was forbidden in early ecclesiastical music, and early theorists called it diabolus in musica ("the devil in music"). But Tartini used it, for this very reason, in his Devil's Trill Sonata for violin.
Though the raw tritone sounds so harsh, it is easily filled out with another tritone to form a diminished seventh. And this, the Oxford Companion to Music notes, "has a luscious effect... The chord is indeed the most Protean in all harmony. In England the nickname has been given it of 'The Clapham Junction of Harmony' - from a railway station in London where so many lines join that once arrived there one can take a train for almost anywhere else. "

― Oliver Sacks , Musicophilia: Tales of Music and the Brain