Tuotilo’s Introit trope Hodie cantandus est nobis includes on Quis est iste puer, quem tam magnis preconiis dignum voci feratis a really interesting juxtaposition of B to Bflat or the famous me against fa dissonance created by conflict of the hexachordum durum and the hexachordum molle. B mi is only separated by one note (A) from B fa. As a modern listener it makes pretty much no sense; as a Renaissance listener, it smacks of the pathos of Purcell or something. I find it hard to accept that these were more or less arbitrary choices on Tuotilo’s part. He has to have chosen this unusual formulation for some reason. Probably the reason’s just that he thought it was a cool sound, but I guess that’s intriguing to me because chromaticism fascinates people as early as c900.