If two propositions are related in such a way that they cannot both be true together, although both may be false together, then such a relation is considered to be:

Two propositions that cannot both be true yet can both be false stand in the contrary relation.

The defining feature is joint truth being impossible.

Yet both may fail together, so joint falsity is allowed.

On the square of opposition this holds between the two universal propositions.

"All S are P" and "No S are P" cannot both be true but can both be false.

A relation with exactly these properties is called contrary.