A diptych is any object with two flat plates which form a pair, often attached by 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.

A wax tablet is a tablet made of wood and covered with a layer of wax, often linked loosely to a cover tablet, as a "double-leaved" diptych. It was used as a reusable and portable writing surface in antiquity and throughout the Middle Ages. Cicero's letters make passing reference to the use of cerae, and some examples of wax-tablets have been preserved in waterlogged deposits in the Roman fort at Vindolanda on Hadrian's Wall. Medieval wax tablet books are on display in several European museums.