Venn diagrams are graphical representations of set theory, which use circles to show the logical relationships of the elements of different sets, using the logical operators (also called in computer parlance “Boolean Operators”) and, or, and not. John Venn (1834–1923) first used them in his 1881 Symbolic Logic, in which he interpreted and corrected the work of George Boole (1815–1864) and Augustus de Morgan (1806–1871). While his attempts to clarify perceived inconsistencies and ambiguities in Boole’s work are not widely accepted, the new method of diagraming is considered to be an improvement. Venn used shading to better illustrate inclusion and exclusion. Charles Dodgson (1832–1898), better known by his pseudonym Lewis Carroll, refined Venn’s system, in particular by enclosing the diagram to represent the universal set.