A diptych (pronounced: dip-tick) is basically a drawing or painting in two parts. The pictures will usually be the same size and format (landscape or portrait), sometimes composed of separate, closely related images or they may be a continuous but divided image.
The word diptych comes from the Greek root 'dis', meaning two, and 'ptykhe', meaning 'fold'. It was the name of the folding writing tablets used in Roman times made from two boards (most commonly wood, but could be bone or metal), that were hinged together, the inner faces being covered with a layer of wax, ready for inscription.