Obverse and reverse

Obverse and its opposite, reverse, refer to the two flat faces of coins and some other two-sided objects, including paper money, flags, seals, medals, drawings, old master prints and other works of art, and printed fabrics. In this usage, obverse means the front face of the object and reverse means the back face. The obverse of a coin is commonly called heads, because it often depicts the head of a prominent person, and the reverse tails.

In fields of scholarship outside numismatics, the term front is more commonly used than obverse, while usage of reverse is widespread.

In publishing, "recto" and "verso" are used for the front and back of pages in books, especially manuscripts, meaning the first side of a leaf encountered by a reader, which will be on the right of an opening for Western manuscripts, but on the left of an opening for those in many Asian and Middle Eastern languages such as Chinese and Arabic. For prints and drawings with material on both sides the one judged as more significant will be the recto.

Obversion

In traditional logic, obversion is a "type of immediate inference in which from a given proposition another proposition is inferred whose subject is the same as the original subject, whose predicate is the contradictory of the original predicate, and whose quality is affirmative if the original proposition's quality was negative and vice versa". The quality of the inferred categorical proposition is changed but the truth value is equivalent to the original proposition. The immediately inferred proposition is termed the "obverse" of the original proposition, and is a valid form of inference for all types (A, E, I, O) of categorical propositions.

In a universal affirmative and a universal negative proposition the subject term and the predicate term are both replaced by their complements:

The universal affirmative ("A" proposition) is obverted to a universal negative ("E" proposition).

The universal negative ("E" proposition) is obverted to a universal affirmative ("A" proposition).