square of opposition
A diagram of the logical relations among A, E, I, and O statements with the same terms.
The square of opposition places the four categorical statements with the same subject and predicate at the corners of a square and labels the relations between them. On the Boolean reading only the diagonals survive: A and O are contradictories, and E and I are contradictories. The traditional square adds contraries, subcontraries, and subalternation, which hold only if the subject class has members.
Examples
"All phones are cheap" and "Some phones are not cheap" sit on a diagonal: exactly one is true.