# Why it is always circle to represent a Set?

When we draw a Venn diagram, we use circle to represent a Set. We can use any closed plane figure but most of the time it is a circle. Why? are there any specialty about that?

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Tradition? I've also seen boxes (i.e. squares) used, BTW. And I draw more oval-like shapes. So I wouldn't even agree it's always a circle.... –  Henno Brandsma Jun 27 '14 at 16:59
Note the circle itself is a set of points in the plane, so you are using a set to represent a set. Prof JF Adams made the point in one of my very first undergraduate lectures, and I've remembered it ever since - "grown up" maths was going to be very different from what went before. –  Mark Bennet Jun 27 '14 at 17:06
I don't like to draw things very much, but I often prefer to use squares. –  Asaf Karagila Jun 27 '14 at 17:49

Circles are easy to draw and conceptually very nice. There is no requirement for anything other than a circle for Venn diagrams with $n \le 3$ classes. Also, the boundaries of two circles intersect each other at not more than two places, which is an important aspect that has connections to some deeper results in mathematics, such as the Jordan Curve Theorem.

However, when you get to $n \ge 4$, you can no longer draw (symmetric) Venn diagrams with circles. $n=4$ requires ellipses, and for prime $n \ge 5$, the shapes get more and more complicated!