A film base is the structural layer of photographic film which acts as a support medium for the film's light-sensitive layers of photographic emulsion. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majority of the thickness of any given film stock. Since the late 19th century, there have been three major types of film base in use: cellulose nitrate, cellulose acetate, and polyester.