  The same relations obtain for modal propositions, for propositions involving binary connectives like “and” and “or”, for quantificational adverbs, and for a range of other operators that can be mapped onto the square in analogous ways utilizing the same notions of contradictory and contrary opposition and with unilateral entailment definable on duals (see Horn 1989). Thus for example we have the modal square below, based on De Interpretatione 21b10ff. and Prior Analytics 32a18–28, where the box and diamond symbols denote necessity and possibility, respectively. As with universal affirmatives and universal negatives, necessity and impossibility constitute contraries: “A priest must marry” and “A priest can’t marry” can both be (and, on the Episcopalian reading, are) false but cannot both be true. “A priest can marry” and “A priest can (if he wants) not marry” are subcontraries; these can be simultaneously true but not simultaneously false. And necessity, as in “A priest must marry”, unilaterally entails its possibility dual counterpart, “A priest may marry”.