Diptychs Church registers, so called because they were originally tablets folded in two leaves (δίπτυχα), wherein, among the early Christians, were recorded the names of bishops and other brethren, whether deceased or living, who were entitled to have their names mentioned in the celebration of the Liturgy from having rendered any signal service to the Church. When a member of the Church was excommunicated, his name was erased from the diptychs. They are still in use in the Greek Church.
The diptych of antiquity consisted of two tablets of wood, ivory, or other substance, which folded together, and contained a coating of wax on the interior. On this wax were written at first private letters. In this case the diptych was bound with a cord, and sealed with wax;. Later, the emperors, consuls, and other magistrates, to celebrate their elevation to office, sent diptychs to their friends, containing on the exterior of the tablets an engraving of their portrait, or of some circumstance connected with their official promotion. They were also employed as public registers. The sacried diptychs contained on one side names of the living, on the other of the dead, which were rehearsed during sacrifices. When Christianity became triumphant, diptychs were used to contain sculptures of religious subjects and scenes. Even the poorest traveler or pilgrim used them to hold the images of sacred persons, before which he bowed in prayer several times a day. In Christian art, a diptych is an altar-paintings in two pieces, which may be folded together, and which contain paintings on both the interior and exterior surfaces. — Siegel, Christl.-kirchliche Alterthümer, 3:259; Bingham, Orig. Ecclesiastes 10:2, 6, and 15:3, 17; Jamieson, History of our Lord in Art, 1:21.