The drawing is an example of a Venn diagram that shows the relationship among three overlapping sets X, Y, and Z. The intersection relation is defined as the equivalent of the logic AND. An element is a member of the intersection of two sets if and only if that element is a member of both
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sets. Venn diagrams are generally drawn within a large rectangle that denotes the universe, the set of all elements under consideration.
In this example, points that belong to none of the sets X, Y, or Z are gray. Points belonging only to set X are cyan in color; points belonging only to set Y are magenta; points belonging only to set Z are yellow. Points belonging to X and Y but not to Z are blue; points belonging to Y and Z but not to X are red; points belonging to X and Z but not to Y are green. Points contained in all three sets are black.
Here is a practical example of how a Venn diagram can illustrate a situation. Let the universe be the set of all computers in the world. Let X represent the set of all notebook computers in the world. Let Y represent the set of all computers in the world that are connected to the Internet. Let Z represent the set of all computers in the world that have anti-virus software installed. If you have a notebook computer and surf the Net, but you are not worried about viruses, your computer is probably represented by a point in the blue region. If you get concerned about computer viruses and install an anti-virus program, the point representing your computer will move into the black area.