What diagrammatic method for syllogisms did Lewis Carroll invent?

Lewis Carroll invented the Carroll diagram, a diagrammatic method for analyzing syllogisms.

Carroll, whose real name was Charles Lutwidge Dodgson, developed the method as part of his work on logic. He introduced an early version in The Game of Logic, published in 1886, and later expanded the approach in his more formal Symbolic Logic. The diagrams represent classes or sets and mark the relationships asserted by logical propositions.

A syllogism contains two premises and a conclusion, such as “All poets are dreamers; all dreamers are imaginative; therefore, all poets are imaginative.” Carroll’s diagrams help a reader place the premises visually and test whether the conclusion follows. His system could also expose fallacies—arguments that appear persuasive but do not logically prove their conclusions.

Carroll’s method is often confused with Venn diagrams because both use visual regions to represent classes. However, Venn diagrams were devised by John Venn and first published in 1880. Carroll developed his own related system, including biliteral and triliteral diagrams, for teaching and solving logical problems. The name “Carroll diagram” therefore refers specifically to his logical notation, not to Venn’s later, more familiar system.