A tritone is the interval of the augmented fourth, consisting of three whole tones, whence the name is derived.
In pure ecclesiastical music the use of the tritonus or Augmented Fourth was strictly forbidden; as was also that of its inversion, the Quinta falsa or Diminished Fifth.
It is scarcely necessary to say that the presence of these intervals is felt, whenever F and B are brought either into direct or indirect correspondence with each other, wheatever may be the Mode in which the contact takes place.
Now, according to the system of Solmisation adpted by guido d’Arezzo, B, the third sound of the Hexachordon durum, was called MI; and F, the fourth sound of the Hexachordon natural, was called FA.
Medieval writers, therefore, expressed their abhorrence of the false relation existing between these two sounds, in the proverb – ‘Mi contra fa est diabolus in muzica.’
When the use of the Hexachords was superseded by a more modern system of immutable Solmisation, F still retained its name of FA, while B took that of the newly added syllable, SI: and the old saw then ran thus: ‘Si contra fa est diabolus in musica.’
In this form it became more readily intelligible to musicians unacquainted with the machinery of the Hexachords; while its signification remained unchanged, and its teaching was as sternly enforced as ever.
That the teaching continues in full force still is proved by the fact that neither Pietro Aron nor any other early writer ever censured the ‘False relation of the Tritone’ more severely than Cherubini, who condemned it, with equal rigor, whether it be used as an element of harmony or of melody.