The history of the Christian altarpiece is difficult to pin down. During the 4th century, it became customary to construct altars directly above the tombs of saints. The only objects allowed on the stone slab were the Bible and the accessories needed to perform the Eucharist. By the 9th century, the pope permitted the display of reliquaries, containers with relics, on the surface of the altar. Prior to this development, the physical traces of saints were hidden from view underneath the structure or inside a niche covered with grating. After 1000 AD, a series of decorative panels called an antependium would be crafted along the entire width of the front. In Italy, the original format of an altarpiece was a detachable, horizontal series of panels hung over the table featuring the Virgin or Christ in a center panel along with saints in others. By the turn of the 13th c., however, priests had begun conducting mass from the front of the altar and a choir screen divided the laity from the clergy, blocking decorations at the high altar from public view. Chapels with side altars originated before the 10th century and were initially used for private ceremonies by clergy. During the 13th c., laypeople venerated particular saints at these devotional altars where altarpieces frequently served as reliquaries. Certain ecclesiastical orders, dependent on alms for sustenance, realized the effectiveness of images to stimulate devotion within the illiterate masses. The polyptych, a tiered structure made of four or more joined panels, was introduced in the 14th c. Its frame typically resembled the architecture on the face of a Gothic church.