The altarpiece used only to be opened on feast-days. For the rest of the time, the wings were closed, so that all that could be seen was the Annunciation. the colours of the open painting are deliberately brighter and more festive than those of the closed shutters. The viewer's full attention is drawn to the large centre panel, in which a divine liturgy is performed around the Lamb of God, Jesus Christ, in a green landscape with a high horizon. an altar stands in the middle on a small green mound, possibly representing Mount Zion. the Lamb stands erect on the altar - alive but bleeding from a wound. It's blood spurts out into a chalice. The allusion is clear - Christ's death and ressurection - his sacrifice for humanity - are commemorated on the altar during every Mass. the scene as a whole referst to the Last supper when Jesus described the wine as his blood, which he was to shed for all humanity to earn the forgiveness of its sins.