Text: The name given by the old contrpuntists to the tritone, which was always to be avoided ­ "mi contra fa est diabolus". It is not as some suppose, the simultaneous sounding of the tonic with the leading note; but, as the notes of the scale of C in old solfaing were named ­ fa, sol, la, fa, sol, la, mi, fa ­ the union or succession of the fa and mi would form the tritone, the leading note in the olds cales always being called mi.

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Source: 125