This interval is exists between those tones, in the octave's tone circle, that are found opposite to each other, and are therefore (in the logarithmic world) approximating to the square root of two (the two of actaval doubling) relative to one another. Critically, the tritone is most significantly found opposite the octave's tonic note, sometimes called do, and therefore lies between fa and sol using the same terminology.

Title Created Date
Danielou's India and the Tritone 07 March 2018
What McClain wrote about Agni as Tritone 07 March 2018