The film gate is the rectangular opening in the front of a motion picture camera where the film is exposed to light (or an opening for showing the film with a projector). The film gate holds the film during exposure through the aperture formed by the shutter. The film gate can be seen by removing the lens and rotating the shutter out of the way. The film is held on a uniform plane at a calibrated distance in the gate by a pressure plate behind the film.
Occasionally, as the film passes through the gate, friction can cause small slivers of celluloid to break off and stick in the side of the opening, or a speck of dust can lodge there. These pieces of debris are called hairs. A "hair in the gate" will remain in front of the film and create a dark line that sticks into the edge of the film frame as the camera is filming a shot. A hair can ruin the shot and is almost impossible to fix in post production without using modern digital removal techniques.
