I sketched out this large diptych, started painting it, and realized it would a stronger composition if I had flipped the 1/3 and 2/3 panels. I could have avoided having to redraw this with a simple thumbnail sketch.
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. When I paint them I try to make each panel stand on its own, but create a better image together.