Propositional logic is a formal system in mathematics and logic. Other names for the system are "propositional calculus" and "sentential calculus". The system is made of a set of propositions. Each proposition has a truth value, being either true or false. Propositions can be represented by capital roman letters such as formula_1, formula_2 and formula_3, and joined together using logical connectives to make new propositions. Examples for logical connectives that are used often are logical and (formula_4), logical or (formula_5), logical if (formula_6), logical if and only if (formula_7) and logical not (formula_8).