Most "Double Roll" film was originally sold as 25 feet of 16mm film. The cameraman would shoot one edge of the film, flip-over the spool in the the camera (in the dark) and then shoot the other edge of the film. After the film was developed, it was split down the center and two ends were glued together to form one 50 foot reel. That is why the little gray plastic reel is so much thinner than the little yellow box.