The MacKinsey axiom of classical modal logic, which is also known as axiom M\textbf{M}M, is given by the schema
□◊ϕ→◊□ϕ \Box \Diamond \phi \to \Diamond \Box \phi □◊ϕ→◊□ϕ
This axiom is important because there is no first-order sentence that characterises the Kripke frames which satisfy it.