Suppose is the set of all reptiles, is the set of all creatures that live in the sea, and is the set of all mammals. We get the Venn diagram:
The region labelled is the intersection of and ; the set of reptiles which live in the sea. Similarly is the set of mammals that live in the sea. Since there is no such thing as an animal which is both a reptile and a mammal, the intersection is empty (the and regions don't cross over each other).
Below we show some examples of animals in each category of the Venn diagram.
For another example, let , , , . A Venn diagram for this situation would look like this: