S5 (modal logic)

In logic and philosophy, S5 is one of five systems of modal logic proposed by 
Clarence Irving Lewis and Cooper Harold Langford in their 1932 book "Symbolic Logic". 
It is a normal modal logic, and one of the oldest systems of modal logic of any kind. It is formed with propositional calculus formulas and tautologies, and inference apparatus with substitution and modus ponens, but extending the syntax with the modal operator "necessarily" formula_1 and its dual "possibly" formula_2.
The axioms of S5.
The following makes use of the modal operators formula_1 ("necessarily") and formula_2 ("possibly").
S5 is characterized by the axioms:
and either:
The (5) axiom restricts the accessibility relation formula_10 of the Kripke frame to be Euclidean, i.e. formula_11.
Kripke semantics.
In terms of Kripke semantics, S5 is characterized by models where the accessibility relation is an equivalence relation: it is reflexive, transitive, and symmetric.
Determining the satisfiability of an S5 formula is an NP-complete problem. The hardness proof is trivial, as S5 includes the propositional logic. Membership is proved by showing that any satisfiable formula has a Kripke model where the number of worlds is at most linear in the size of the formula.
Applications.
S5 is useful because it avoids superfluous iteration of qualifiers of different kinds. For example, under S5, if "X" is necessarily, possibly, necessarily, possibly true, then "X" is possibly true. Unbolded qualifiers before the final "possibly" are pruned in S5. While this is useful for keeping propositions reasonably short, it also might appear counter-intuitive in that, under S5, if something is possibly necessary, then it is necessary.
Alvin Plantinga has argued that this feature of S5 is not, in fact, counter-intuitive. To justify, he reasons that if "X" is "possibly necessary", it is necessary in at least one possible world; hence it is necessary in "all" possible worlds and thus is true in all possible worlds. Such reasoning underpins 'modal' formulations of the ontological argument.