Truth tables show the values, relationships, and the results of performing logical operations on logical expressions. The logical operators are not, and, or, conditional, and biconditional. The logical expressions[?] are necessarily true[?], contingently true[?], or necessarily false[?].
The column headings on a truth table show the input variables and output expressions. The rows show each possible combination of inputs, one combination per row, and the outputs that result from each combination of inputs.
Truth tables are usually limited to bivalent logic systems[?] where only two truth values are possible, true or false.
For example, take two terms, A and B, and the logical operator "and" (∧), signifying the conjunction "A and B". In common English, if A is true and B is true, then the conjunction "A and B" is true; under all other possible assignments of truth values to A and B, the conjunction is false. This relationship is defined as follows:
A B A ∧ B T T T T F F F T F F F F
In a bivalent logic system, all the operators can be explicitly defined this way. For example, the not (~) relationship is defined as follows:
A ~A T F F T
The or (∨) relationship is defined as follows:
A B A ∨ B T T T T F T F T T F F F
Compound expressions can be constructed, using parenthesis to denote precedence. The negation of conjunction [ ~( A ∧ B ) ], is depicted as follows:
A B A ∧ B ~( A ∧ B ) T T T F T F F T F T F T F F F T
Truth tables can be used to prove logical equivalence. The truth table for the disjunction of notA and notB is:
A B ~A ~B A ∧ B ~(~A ∨ ~B) T T F F T T T F F T F F F T T F F F F F T T F F
Because the enumeration of possible truthvalues for A and B yields the same truthvalue under both ( A ∧ B ) and ~(~A ∨ ~B), the two are logically equivalent, and may be substituted for each other. This equivalence is one of DeMorgan's Laws).
An example of a truth table in logic:
P  Q  P & Q  P ∨ Q  P xor Q  P → Q  P <> Q T  T  T  T  F  T  T T  F  F  T  T  F  F F  T  F  T  T  T  F F  F  F  F  F  T  T
Key:
Johnston diagrams, similar to Venn diagrams or Euler Diagrams, provide a way of visualizing truth tables. An interactive Johnston diagram illustrating truth tables is at LogicTutorial.com (http://logictutorial.com)
See also Connective, Logical calculus
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