Diptych
A diptych (/ˈdɪptɪk/, DIP-tick) is an object made up of two flat plates or panels that form a pair, often joined by a hinge. In the ancient world, for example, the standard notebook or school exercise book often took the form of a diptych: two plates with recessed surfaces filled with wax. Writing was done by scratching the wax surface with a stylus. When the notes were no longer needed, the wax could be gently heated and smoothed for reuse. Ordinary diptychs usually had wooden frames, while more luxurious examples were made from costlier materials.