In the past, double exposing film was one of the most popular special effects used in photography, and often could be hit or miss. The idea was to combine two (or more) photos into the same image.
In the days of film, this was done by taking two photos onto the same frame by shooting a second frame without advancing the film. Or, in the case of large format cameras, simply leave the film holder in place for a second shot.

Many 35 mm film cameras could do this by using a special function to allow the shutter to be cocked without advancing the film. You could also do this on cameras not designed with this function by carefully holding the rewind knob steady, pressing the rewind release button, and then advancing the film lever.

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