1. Most things people call Venn diagrams are not Venn diagrams.
Lots of them are actually Euler diagrams. A Venn diagram has to show all possible logical relationships between sets, even if they’re not things that exist; an Euler diagram doesn’t.
2. If you want some added explanation, here are more charts than promised in the headline.
Venn diagrams are (roughly) a very specific type of Euler diagram. For example, a Venn diagram has to show the overlap between two sets, even if there’s nothing in that overlap (shown by shading it in black, as above).
3. While a Euler diagram can just show that they don’t overlap:
Venn diagrams also have to show every part of every set overlapping with every other at some point, which makes them really messy compared to Euler diagrams.
5. And of course some things described as “Venn diagrams” aren’t even Euler diagrams. They’re just… circles.
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