A logical operator joins two propositions to form a new, complex, proposition. (If you have not read about propositions, you should do so now. Follow the Next links until you return to this page.) The truth value of the new proposition is determined by the truth values of the two propositions being joined and by the operator that joins them.
Any two propositions P and Q can be joined with the biconditional operator, producing the new, complex, proposition:
P if and only if Q
The proposition P if and only if Q is true if and only if both P and Q are true, or if both P and Q are false. It is false only when one of them is true and the other false.