A diptych is any object with two flat plates attached at a 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.
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, 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 p