A diptych (; from the Greek δίπτυχον, di "two" + "fold") is any object with two flat plates attached at a hinge. In particular the standard notebook and school exercise book of the ancient world was the diptych consisting of a pair of such plates that were wax tablets, with on their inside faces a recessed space filled with wax. This took writing made with a pointed 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 luxury ones were made in more expensive materials.