Countries with weak and/or developing economies generally use foreign exchange controls to limit speculation against their currencies.
This leads to a situation where the actual demand for foreign currency is greater than that which is available on the official market.
For instance, many western European countries implemented exchange controls in the years immediately following World War II.
The measures were gradually phased out, however, as the post-war economies on the continent steadily strengthened; the United Kingdom, for example, removed the last of its restrictions in October 1979.
After a very short interruption, exchange controls were restored in 1968, relaxed in 1984, and finally abolished in 1989.