The drawings in fifteenth-century model books functioned as a stock of exemplars that could be reused by an artist. Drawn on prepared paper in metalpoint, the images in model books were highly finished. Because of their frequent use, only a small number of these drawings survived, but aspects of this practice are known to have persisted in early sixteenth-century Netherlandish workshops. Sometime about 1500 Gerard David copied four heads from Jan van Eyck's Ghent Altarpiece. In his metalpoint drawing, based on the popes and bishops shown among the Holy Martyrs on the altarpiece, David carefully positioned the heads so as not to overlap, thus the artist could later quickly integrate them into a painting.