 Many well-known modal logics \(S\) are not only complete with respect to an appropriate class of frames, but even strongly complete. In order to explain strong completeness, we need the notion of derivability from a set of assumptions. A formula \(A\) is derivable from the set of assumptions \(\Gamma\) in a modal logic \(S\), written as \(\Gamma \vdash A\), if \(A\) is in \(\Gamma\) or \(A\) follows from formulas in \(\Gamma\) and axioms of \(S\) by applications of Modus Ponens and the Generalization Rule. Here, the Generalization Rule can only be applied to derivations without assumptions (see Hakli and Negri 2010).