In just about any traditional art form you work from the bottom up. You paint a background, then a foreground, etc. The last thing you do is the 'topmost' layer of paint, and the most clearly visible. When working on a cel, the first thing you do is the topmost layer, and the background is the very last thing applied. Because you are working on a clear surface, when the image is complete it is the bottom layer that you see, inverted from the image you painted and completely backwards and utterly different from the 'worked' side with paint on it. Observe: |