In a vintage shop, or in the home of an older relative, you may have seen one of these:
Here’s how it’s operated:
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That’s called a perpetual flip calendar, versions of which were first patented in the U.S. in the 1920s.
The guy behind the “Measured Workshop” YouTube channel wanted to figure out how they worked, so he downloaded the patent drawings (here and here), then built his own out of wood. In the video below he shows you the build process, then demonstrates how they work. (The video’s edited a bit on the slow side, so if you’re impatient, you can skip ahead to about 5:30 to see how the mechanism works.)