A combination of a conditional statement and its converse written in the "if and only if" form. Both the conditional and the converse must be true before writing this statement.

An angle is 90° if and only if it is a right angle.

Explanation:

The statement is: If an angle is 90°, then it is a right angle.

The converse of this statement would be:

If an angle is a right angle, it is 90°.

Clearly, both the conditional and converse of this statement is true.