Specifies an axiomatization of the system S13 of modal logic. Referenced in Cocchiarella & Freund "Modal Logic: an Introduction to its Syntax and Semantics", Oxford University Press, 2008.

Discusses alternative interpretations of the modal operators, for the modal propositional logic S5.

It is shown that the modally first-degree formulas of quantificational S5 constitute a reduction class. This is done by defining prenex normal forms for quantificational S5, and then showing that for any formula A there is a formula B in prenex normal form, such that B is modally first-degree and is provable if and only if A is provable.