Given a Disjunctive in the form _Either A is B or C is D_, we may write for its Obverse--_In no case is A b, and C at the same time d_.

But no Converse or Contrapositive of such a Disjunctive can be obtained, except by first casting it into the hypothetical or categorical form.
The reader who wishes to pursue this subject further, will find it elaborately treated in Dr.Keynes' _Formal Logic_, Part II.; to which work the above chapter is indebted..