Venn Diagram

http://en.wikipedia.org/wiki/Venn_diagram#cite_note-0

A Venn diagram is constructed with a collection of simple closed curves drawn in the plane. The principle of these diagrams is that classes or sets be represented by regions in such relation to one another that all the possible logical relations of these classes can be indicated in the same diagram. That is, the diagram initially leaves room for any possible relation of the classes, and the actual or given relation, can then be specified by indicating that some particular region is null or is notnull.[1]

Venn diagrams normally consist of overlapping circles. For instance, in a two-set Venn diagram, one circle may represent the group of all wooden objects, while another circle may represent the set of all tables. The overlapping area (intersection) would then represent the set of all wooden tables. Shapes other than circles can be employed and this is necessary for more than three sets.

[1] Clarence Irving Lewis (1918). A Survey of Symbolic Logic. Republished in part by Dover in 1960. p.157.