 The least normal modal logic is called \(K\) and can be axiomatized by the Hilbert-style calculus with axioms the tautologies of classical logic and the formulas \(\Box(\phi \rightarrow \psi) \rightarrow(\Box \phi \rightarrow \Box \psi)\), and with rules of inference Modus Ponens and Modal Generalization. Note that since we use schemas in the presentation of the axioms, the set of derivable formulas is closed under the Substitution  rule.