An optimal currency area is often larger than a country. For instance, part of the rationale behind the creation of the euro is that the individual countries of Europe do not each form an optimal currency area, but that Europe as a whole does. The creation of the euro is often cited because it provides the most modern and largest-scale case study of an attempt to identify an optimum currency area, and provides a comparative before-and-after model by which to test the principles of the theory.