Full Idea
Square of Opposition: horizontals - 'contraries' can't both be true, and 'subcontraries' can't both be false; verticals - 'subalternatives' have downwards-only implication; diagonals - 'contradictories' have opposite truth values.
Gist of Idea
Square of Opposition: not both true, or not both false; one-way implication; opposite truth-values
Source
Aristotle (On Interpretation [c.330 BCE], Ch.12-13)
Book Reference
Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.62
A Reaction
This is still used in modern discussion (e.g. by Stalnaker against Kripke), and there is a modal version of it (Fitting and Mendelsohn p.7). Corners read: 'All F are G', 'No F are G', 'Some F are G' and 'Some F are not G'.
Related Idea
Idea 15879 The Square of Opposition has two contradictory pairs, one contrary pair, and one sub-contrary pair [Harré]