Full Idea
The system S5 has the 'reflexive', 'symmetric' and 'transitive' conditions imposed on its accessibility relation - that is, every world is self-accessible, and accessibility is mutual, and it carries over a series of worlds.
Gist of Idea
System S5 has the 'reflexive', 'symmetric' and 'transitive' conditions on its accessibility relation
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.19
A Reaction
S5 has total accessibility, and hence is the most powerful system (though it might be too powerful).