No, the sets and subsets can be any shape, but circles and rectangles are neater. Also, circles or rectangles will usually intersect in only one part whereas very wriggly shapes can have multiple intersections.
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you can have 2-3 circles, but for heavy information you can have 5 circles. I'm not sure if there can be more i would have to look into that.
well in fact the difference is one has 2 circles and one has 3
Venn diagrams were introduced in 1880 by John Venn (1834-1923) in a paper entitled On the Diagrammatic and Mechanical Representation of Propositions and Reasonings in the "Philosophical Magazine and Journal of Science", about the different ways to represent propositions by diagrams. Venn himself did not use the term "Venn diagram" and referred to his invention as "Eulerian Circles."
A Venn diagram gets unwieldy with a large number of observations.It is impossible (in 2-d) to have more than three categories (circles).