Very simply, a diptych is a drawing or painting in two parts. The format of the pictures may be landscape or portrat, but they will usually be the same size. It is sometimes a continuous but divided image, or may be composed of separate, closely related images. Often you'll see portraits of a couple with one person in each panel.
The word diptych comes from the greek root 'dis', meaning two, and 'ptykhe', meaning 'fold', and was the name of the folding writing tablets used in Roman times. Two boards, most commonly wood, but also bone or metal, were hinged together, the inner faces covered with a layer of wax which could be inscribed.
Common Misspellings: dyptich, diptich