A recent discovery has brought to light a stave of Gregorian music inscribed on the wall to the left of the sacristy door. It proves to be the remnant of a fresco. The pigmentation of the sinopite has passed through the plaster, so to stain the wall. A piece of Gregorian music, three meters long, and one meter high, is thus visible to us. Besides the notes, also the letters MINO are well distinguished, almost certainly the last four letters pertaining to the word DOMINO, written below the stave. An undulating file of notes precedes the letters to indicate the jubilus – a group of notes chanted on the same syllable (O). The opening-up of the sacristy door in the 15th century has cancelled the first part of the music stave. What melody does it now conceal?