The angels on the Ghent Altarpiece (Van Eyck) are singing plainchant and/or polyphony. If it is plainchant, they could be responses from the period after Easter. These responses make explicit references to the book of Revelations - which makes sense because the Ghent Alterpiece is full of references to the Apocalypse. The combination of these chants with two-voices Agnus-Dei fragments from masses by Dufay, Ockeghem, Desprez and others, make for a series of small and beautiful vignettes. Triptychs.