Full Idea
Modal logics were, for a long time, studied in terms of axiom systems. The advent of possible worlds semantics made it possible to study them in a semantic way as well.
Gist of Idea
Modal logics were studied in terms of axioms, but now possible worlds semantics is added
Source
Rod Girle (Modal Logics and Philosophy [2000], 6.5)
Book Reference
Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.92