What is "cue mark"

n. A visual indicator, placed in an upper corner of a frame of a motion picture, to signal the projectionist to change reels

A cue mark, also known as a cue dot, a changeover cue or simply a cue is a visual indicator used with motion picture film prints, usually placed on the right-hand upper corner of a frame of the film. Cue dots are also used as a visual form of signalling on television broadcasts.

A pair of cue marks is used to signal the projectionist that a particular reel of a movie is ending, as most movies come to theaters on several reels of film lasting about 14 to 20 minutes each. The marks appear in the last seconds of each reel; the first mark, known as the motor cue, is placed about 8 seconds before the end of the picture section of the reel. The second mark, known as the changeover cue, is placed about 1 second before the end. Each mark lasts for 4 frames (0.17 seconds).

Coded Anti-Piracy is a different kind of mark, used for watermarking to detect and prevent copyright infringement.