 The most commonly studied Hilbert systems have either just one rule of inference modus ponens, for propositional logics or two with generalisation, to handle predicate logics, as well and several infinite axiom schemes.Hilbert systems for propositional modal logics, sometimes called Hilbert-Lewis systems, are generally axiomatised with two additional rules, the necessitation rule and the uniform substitution rule.A characteristic feature of the many variants of Hilbert systems is that the "context" is not changed in any of their rules of inference, while both natural deduction and sequent calculus contain some context-changing rules.