A diptych (; from the Greek δίπτυχον, di "two" + ptychē "fold") is any object with two flat plates which form a pair, often attached by hinge. For example, the standard notebook and school exercise book of the ancient world was a diptych consisting of a pair of such plates that contained a recessed space filled with wax. Writing was accomplished by scratching the wax surface with a stylus. When the notes were no longer needed, the wax could be slightly heated and then smoothed to allow reuse. Ordinary versions had wooden frames, but more luxurious diptychs were crafted with more expensive materials. Art As an art term a diptych is an artwork consisting of two pieces or panels, that together create a singular art piece these can be attached together or presented adjoining each other. In medieval times, panels were often hinged so that they could be closed and the artworks protected. In Late Antiquity, ivory notebook diptychs with covers carved in low relief on the outer faces were a significant art-form: the "consular diptych" was made to celebrate an individual's becoming Roman consul, when they seem to have been made in sets and distributed by the new consul to friends and followers. Others may have been made to celebrate a wedding, or, perhaps like the Poet and Muse diptych at Monza, simply commissioned for private use. Some of the most important surviving works of the Late Roman Empire are diptychs, of which some dozens survive, preserved in some instances by being reversed and re-used as book covers. The largest surviving Byzantine ivory panel (428 mm × 143 mm), is a leaf from a diptych in the Justinian court manner of c. 525–50, which features an archangel. From the Middle Ages many panel paintings took the diptych form, as small portable works for personal use; Eastern Orthodox ones may be called "travelling icons". Although the triptych form was more common, there were also ivory diptychs with religious scenes carved in relief, a form found first in Byzantine art before becoming very popular in the Gothic period in the West, where they were mainly produced in Paris. These suited the mobile lives of medieval elites. The ivories tended to have scenes in several registers (vertical layers) crowded with small figures. The paintings generally had single subjects on a panel, the two matching, though by the 15th century one panel (usually