Tenor structure of the motet Nuper rosarum flores by Guillaume Dufay (1436). Top: Preexisting Gregorian cantus firmus, "Terribilis est locus iste" ('This place is awesome'), from the introit for the consecration of churches [1], here transposed from d to g. Second staff: Tenor as notated in mensural notation, with the four mensuration signs defining the diminution canon. Below: schema of execution with fourfold diminution. The tenor is performed four times, each totalling 28 bars, but each time measured in a different time signature, resulting in proportions 6:4:2:3. Each part consists of 14 bars of rests in the tenor and counter-tenor, and 14 bars sung by all four voices.
Source: Dtv-Atlas zur Musik, Bärenreiter 1977, vol 1, p.238f.
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Future Perfect at Sunrise