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Full Idea
Normal propositional modal logics derive from the minimal system K: wffs of PC are axioms; □(p⊃q)⊃(□p⊃□q); uniform substitution; modus ponens; necessitation (α→□α).
Gist of Idea
Normal system K has five axioms and rules
Source
Max J. Cresswell (Modal Logic [2001], 7.1)
Book Ref
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.137