This interval, exactly in the middle of an octave on the piano keyboard, has a distinguished history of heresy.

The churchman, Theodor, of Arezzo, Italy, about 900, was codifying much of what we now recognize as musical notation. Among his gifts to us, was the diabolus in musica.

Theodor felt it to be the devil in music because it wouldn't resolve either up to the perfect fifth, or down to the perfect fourth; it just stuck there, hanging in space. Such things weren't allowed in Church music.

When I started doing blues improvization on the piano, I loved the indeterminate quality of the diabolus in musica; it was the perfect sound for my ear.

Today, we call it, analytically, the tritone.