| A little-known part of film terminology, the change-over mark is printed onto certain film frames as a visual cue to the film projectionist at the cinema. Here's how they work and why they're needed:
Most film reels hold about twenty minutes worth of film, so a typical movie will be consist of several such reels which must, of course, be viewed in the correct sequence. There is a slight overlap between the final frames on one film reel and the first frames on the next.
To ensure a smooth transition from one reel to the next, two film projectors are used in the projection booth. The change-over mark appears on screen briefly a fixed time before the end of a reel and is a cue for the projectionist to start the other projector, (holding the next reel), such that they are both synchronised and projecting the same thing until the first reel runs out. The projectionist then has twenty minutes to load and cue the third reel on the 'first' projector. Most people don't notice the change-over mark and, if they do, they probably put it down to a blemish in the film print. Keep an eye out for the change-over mark next time you see a movie! | 
