A method of displaying relations between subsets of some universal set. The universal set E is represented by the interior of a rectangle, say, and subsets of E are represented by regions inside this, bounded by simple closed curves. For instance, two sets A and B can be represented by the interiors of overlapping circles, and then the sets A ∪ B, A ∩ B and A\\B = A ∩ B′, for example, are represented by the shaded regions shown in the figures.
Given one set, A, the universal set is divided into two disjoint subsets A and A′, which can be clearly seen in a simple Venn diagram. Given two sets A and B, the universal set E is divided into four disjoint subsets A ∩ B, A′ ∩ B, and A ∩ B′ and A′ ∩ B′. A Venn diagram drawn with two overlapping circles for A and B clearly shows the four corresponding regions. Given three sets A, B and C, the universal set E is divided into eight disjoint subsets A ∩ B ∩ C, A′ ∩ B ∩ C, A ∩ B′ ∩ C, A ∩ B ∩ C′, A ∩ B′ ∩ C′, A′ ∩ B ∩ C′, A′ ∩ B′ ∩ C, and A′ ∩ B′ ∩ C′, and these can be illustrated in a Venn diagram as shown here.
Venn diagrams can be illustrative but should generally be avoided for careful proofs, because a diagram may only illustrate a special case. Four general sets, for example, should not be represented by four overlapping circles because they cannot be drawn in such a way as to make apparent the 16 disjoint subsets into which E should be divided. See also truth tables.