Satanic comma

The satanic comma (monzo: [-1054 665⟩) is the difference between 666 perfect fifths (octave-reduced) and a single perfect fifth. Equivalently, it is the difference between 665 perfect fifths (octave-reduced) and the unison – but that would not be as devilishly intriguing.

This difference is inaudible, at only 0.076 ¢.

It is tempered out in 665edo and its multiples (1330edo, etc.).