In truth-functional propositional logic, commutation, or commutativity refer to two valid rules of replacement. The rules allow one to transpose propositional variables within logical expressions in logical proofs. The rules are: ( P ∨ ∨ Q ) ⇔ ⇔ ( Q ∨ ∨ P ) and ( P ∧ ∧ Q ) ⇔ ⇔ ( Q ∧ ∧ P ) where " ⇔ ⇔ " is a metalogical symbol representing "can be replaced in a proof with".