In this video Hendrik Vanden Abeele talks about folio 9 verso of the 1481 Ghent antiphonary. This folio comprises the end of Matins at the Third Sunday of Advent. There is a 16th-century addition at the end of the folio.
He also talks about the melody of the verses of responsories. The melody is always the same but adapted to the text, changing with the mode of the responsory. This is quite an important episode for it is explained how several things (like the mode theory, reciting notes, etc.) work in plainchant, and they are explained in a very clear and perceptible way.