Why two projectors?

Obviously, a film is longer than 20 minutes, so how do you watch a two-hour film without stopping it to change the reels? The answer is to use two projectors and do a 'changeover'.
Cue Dots:
When the film is playing in Projector 1, the projectionist will be watching the screen out of their viewing porthole. Eight seconds before the end of the reel, printed on four consecutive frames of film, is a 'cue dot'.
Four frames of film means the cue dot appears on screen for 0.17 seconds.
"Motor!"
This first cue dot is the 'motor cue'. The projectionist will call out to their colleague, manning Projector 2,
"motor!" On hearing this, Projector 2's motor will be started.
"Over!"
Meanwhile, the projectionist continues to watch the screen for the next cue dot, positioned one second from the end of the reel. When this appears, they'll call out "over" and will hit the changeover switch. This simultaneously closes the dowser on Projector 1 and opens the dowser on Projector 2.
The audience won't notice a thing.
The film from Projector 1 can now be removed and rewound, and the new reel laced up ready for the next changeover.