Propositional logic is a branch of logic that studies ways of combining or altering statements or propositions to form more complicated statements or propositions. A proposition is a declarative sentence that is either true or false, but not both. It is the basic building block of logic and can be thought of as a statement that is capable of having a truth-value. Propositions are often related to closed formulae or logical sentences to distinguish them from what is expressed by an open formula. In propositional logic, logical operators are used to combine propositions to form compound propositions. Examples of logical operators include negation, conjunction, and disjunction.