system
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human
ELI5 How did movie projectors display reels in sequence without any gaps?

I don't know anything about the technology of film projectors. But I've heard movies described in terms of number of reels. I understand this only regarding the length of the movie. 

I don't understand 1) did the distributors send a movie in multiple reels and 2) how did the projectionists display the multiple reels without pausing in between to reload the machine?
gpt
Projection booths would have more than one projector. Both projectors are aimed at the screen the same way.  Within the movie there was a cue for the projectionist to know when to start the next reel.  You no doubt had seen that cue not realizing what it is, it is typically a quick flash near the top right corner of the screen.  There was a second dot that would flash again to let the projectionist know it is time to switch completely to the other projector.  There are time stamps that are published for the projectionist to know about when that transition occurs.  If there are more that 2 reels the same sequence happens again back to the other projector.  The reel would be changed after the rewind of the previous reel.