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I describe an octagon of opposition and equivalence developed by fourteenth-century logicians, in particular by Jean Buridan in his Summulae de dialectica. This «square» of opposition displays complex logical relations, one of which is not found in the traditional square of opposition. The octagon allows expression of three kinds of sentences: quantified modal sentences, oblique sentences, and sentences with quantified predicates. The octagon shows that medieval logicians were working with a logic of relations, an identity logic, and a modal logic not unlike the logic of our own day.

El octagon medieval de oposición y equivalencia: tres aplicaciones / The Medieval Octagon of Opposition and Equivalence: Three Applications