In traditional logic the obverse of a proposition is one logically equivalent to it, formed by the operation of obversion. For the four basic forms (see syllogism) this transforms

1. (i) ‘All As are B’ into ‘No As are non-B’;

2. (ii) ‘No As are B’ into ‘All As are non-B’;

3. (iii) ‘Some As are B’ into ‘Some As are not non-B’; and

4. (iv) ‘Some As are not B’ into ‘Some As are non-B’.