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Frame lines shown in red on a "full-frame" negative, and on a "hard-matted" 1.85:1 projection print, both on 35 mm film.
A frame line is the unused space that separates two adjacent images, or film frames, on the release print of a motion picture. They can vary in width; a 35mm film with a 1.85:1 hard matte has a frame line approximately 8 millimeters (0.3 inches) high, whereas both a full frame negative and the anamorphic format have very narrow frame lines, with the frames very close together. When a film is properly projected, the frame lines should not be visible to the audience and are typically cropped out in projection with an aperture mask.