The dowels holding the planks of the Andrea DI Bartolo polyptych together were fashioned from whittled oak twigs. Tiny spots of gesso (and in one instance crystals of azurite) on the dowels indicate that they were in close proximity to the panels while the latter were being gessoed and painted. It was only after each panel was completed that they were joined together with dowels placed in holes drilled along the sides of the panel, and further reinforced with horizontal battens the width of the altarpiece, nailed to the back.