This short hommage to Brian Ferneyhough is constructed around the interval formed, with the help of a quarter tone, between the augmented fourth and the fifth. This interval is derived from Brian Ferneyhough's initials, which spell the notes Bb-F in German, but B-F in English; the use of quarter tones allowed me to reach a middle point between the tritone and the fifth, and I explore various harmonic constructions based on this interval, particularly in combination with the more traditional tritone and perfect fifth. This interval is also at the basis of the more melodic section in the middle of the piece, where it is used as limits for most melodic contours. Premiered by the Nieuw Ensemble in Amsterdam in Jan. 1993, cond. Ed Spanjaard, on the occasion of BrianFerneyhough's 50th birthday.