John Venn never called his overlapping circles Venn diagrams
John Venn introduced his diagrams in an 1880 paper but referred to them as Eulerian circles, crediting an older tradition. He had met Euler's diagrams in 1862 and only later hit on his version while adapting them to Boolean logic. The name Venn diagram came afterwards, used by Clarence Irving Lewis in 1918.
The difference matters. An Euler diagram shows only the relationships that actually occur. A Venn diagram draws every possible overlap among its sets, including regions that might be empty, which then get shaded. Picture circles for dairy products and for cheeses: the Venn version includes a zone for cheeses that are not dairy, while the Euler version simply nests one circle inside the other. With two circles for two-legged creatures and flying ones, parrots fall in the overlap, penguins and humans in the legs-only part, mosquitoes in the flying-only part, and whales and spiders outside both.
The idea had a long prehistory. Overlapping circles such as Borromean rings were common in the Middle Ages, and diagrams in the Euler tradition trace back to the 16th century, developed by Erhard Weigel, his student Gottfried Wilhelm Leibniz and others, before Euler and later Kant's followers spread them. Venn saw his diagrams as a teaching aid, and Lewis Carroll included Venn's method in his own logic book in 1896.
Drawing more than three sets gets tricky. Venn himself devised an elegant four-set diagram from ellipses. Anthony Edwards later built diagrams for many sets by dividing a sphere, one curve winding like the seam of a tennis ball, and projecting the result into cogwheel shapes; he worked them out while designing a stained-glass window commemorating Venn. Mathematicians have since proved that a diagram of n sets with n-fold rotational symmetry exists exactly when n is prime, a result completed in 2003.
The circles entered schools with the new math movement of the 1960s and later spread into reading lessons. Online, they became a comedy format, from Jessica Hagy's hand-drawn index cards on her blog Indexed, named by Time among the best blogs of 2007, to a viral 2018 chart comparing preachers, DJs and bank robbers.
Source: Venn diagram
