Consider a family of simple (Jordan) curves that intersect pairwise in finitely many points. Let be either the interior or the exterior of . The family is called independent if

for each of the possible choices of the . If, moreover, each of the sets is connected, the independent family is called a Venn diagram. An independent family or Venn diagram is called simple if no three curves have a common point [1, p. 104].

An example with equilateral triangles is from [1, p. 107].