A cel, short for celluloid, is a transparent sheet of plastic (usually acetate) on which objects are drawn or painted for animation. Generally, the characters are drawn on cels and laid over a static background drawing. This reduces the number of times an image has to be redrawn and enables studios to split up the production process to different specialised teams. Using this taylorised way to animate has made it possile to produce films much more cost-effectively. The invention of the technique is generally attributed to Earl Hurd , who patented the process in 1914.
The outline of the images are drawn on the back of the cel. The colors are also painted on the back to eliminate brushstrokes. Traditionally, the outlines were hand-inked but now they are almost exclusively xerographed on.
CEL is also the abbreviation for Centre d'Essais des Landes, a rocket test facility near Biscarosse, France.