E.O.
(having the same matter) are said to stand in Opposition: and Logicians represent these relations by a square having A.I.

E.O.at its corners: A.

Contraries E.
S Co s S u nt e u b ra i b a di r a l ct o l t ct o t e di r e r ra i r n nt e n s Co s s I.

Sub-contraries O.
As an aid to the memory, this diagram is useful; but as an attempt to represent the logical relations of propositions, it is misleading.

For, standing at corners of the same square, A.and E., A.and I., E.and O., and I.and O., seem to be couples bearing the same relation to one another; whereas we have seen that their relations are entirely different.