A tautology is a compound statement that is true for ALL possible truth values of its components. Examples: p OR ~p, (p AND (p => q)) => q, (p => q) OR (q => p). A contradiction is always false for all truth values. Examples: p AND ~p. A contingency is true for some assignments and false for others — most compound statements are contingencies. Key tautologies to recognize: p OR ~p (law of excluded middle), ((p => q) AND p) => q (modus ponens), ((p => q) AND ~q) => ~p (modus tollens), (p => q) <=> (~q => ~p) (contrapositive equivalence). To verify, construct the truth table — if the final column is all T, it is a tautology.