Book excerpt: In the mid-15th century, when the tradition styles and techniques of the Middle Ages were yielding to the new influences of the Renaissance, the altarpieces of cathedrals and major churches reached a degree of elaboration never seen before. For a century or so altarpieces had been constructed so that they could be closed or open (for saints' days and festivals), often in three parts (triptychs), with two wings folding over the centre. This scheme was now expanded: panels were arranged sometimes in two tiers which could open separately. The three-part stucture could grow to five and even seven. In the most extreme case, Grunewald's Isenheim Altarpiece, there was an unprecedented number of possibilities - a sort of theological hierarchy, with panels opening to reveal deeper and deeper mysteries.