 The opening lines of Dufay's text refers to Pope Eugene IV's gift to the cathedral, and to the city of Florence, of a golden rose to decorate the high altar—a gift made the week before the dedication .The two homographic tenors, which define the overall structural plan of the piece, are both based on a Gregorian cantus firmus melody taken from the introit for the consecration of churches, "Terribilis est locus iste" ('Awesome is this place', Genesis 28