Venn diagram, graphical method of representing categorical propositions and testing the validity of categorical syllogisms, devised by the English logician and philosopher John Venn (1834–1923). Long recognized for their pedagogical value, Venn diagrams have been a standard part of the curriculum of introductory logic since the mid-20th century.

Venn introduced the diagrams that bear his name as a means of representing relations of inclusion and exclusion between classes, or sets. Venn diagrams consist of two or three intersecting circles, each representing a class and each labeled with an uppercase letter. Lowercase x’s and shading are used to indicate the existence and nonexistence, respectively, of some (at least one) member of a given class.

Two-circle Venn diagrams are used to represent categorical propositions, whose logical relations were first studied systematically by Aristotle. Such propositions consist of two terms, or class nouns, called the subject (S) and the predicate (P); the quantifier all, no, or some; and the copula are or are not. The proposition “All S are P,” called the universal affirmative, is represented by shading the part of the circle labeled S that does not intersect the circle labeled P, indicating that there is nothing that is an S that is not also a P. “No S are P,” the universal negative, is represented by shading the intersection of S and P; “Some S are P,” the particular affirmative, is represented by placing an x in the intersection of S and P; and “Some S are not P,” the particular negative, is represented by placing an x in the part of S that does not intersect P.