The vast majority of commercial films are projected digitally today with no 35 mm film used during the entire filming or projecting process. But for decades, films were photographed on 35 mm film and were project on reel to reel film projectors. Although individual projectors will vary, in general you have two film reels on a projector. The upper magazine, called the feed reel, is where the full film reel is placed. It is unwound and fed through the film gate where the light and lenses project the image onto the screen. After passing through the film gate, the film strip is then wound onto an empty reel on the lower magazine, called the takeup reel. So all of the film started in the upper magazine and made its way to the lower magazine.