A diagram showing the relations among categorical propositions is known as the traditional square of opposition [2, p. 217]. Two propositions are contradictories if one is the negation of the other. Propositions A and O, and propositions I and E are contradictories. Two propositions are contraries if they cannot both be true. Two propositions are subcontraries if they cannot both be false. Proposition A is called superaltern, I is called subaltern, and the corresponding relation is called subalternation. The same definitions are applied to E and O [2, pp. 214–217]. Under the assumption that class is not empty, propositions A and E are contraries, propositions I and O are subcontraries, and superaltern implies subaltern.
 L. Carroll, Symbolic Logic and The Game of Logic, New York: Dover Publications, 1958.
 I. M. Copi and C. Cohen, Introduction to Logic, 9th ed., New York: Macmillan Publications, 1994 pp. 214–218.
 J. M. Bocheński, A History of Formal Logic, 2nd ed. (I. Thoma, trans. and ed.), New York: Chelsea Publishing Company, 1970 p. 235.