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lonelysandwich:

An actual tritone is a musical interval that spans three whole tones, and because it normally sounds horrible and dissonant and creates tension to be resolved into a perfect 5th, it’s historically known as Diabolus in Musica, or “the Devil’s interval” and was even suppressed by the Church in the Middle Ages.

Oh, how I love me a good tritone. It’s exactly half an octave, and thus the only interval that is its own inversion. It’s equivalent to either an augmented fourth or a diminished fifth.

You can use tritones to construct an auditory illusion in which the same sequence of tones is heard as ascending by some people and descending by others, and which way you’ll hear it is culturally determined. The consistency with which people resolve the paradox one way or the other for different pitches is evidence that absolute pitch is a not a rare gift but a common latent ability that can be learned.

In one particular Rush instrumental, a two-note theme consisting of the Morse code for Toronto Pearson International Airport appears in the introduction, the lower note representing the dits and the upper note standing in for the dahs. The interval between those two notes? A tritone.