The Universal Postal Union (UPU) a international organization that coordinates postal policies between member nations, and hence the world-wide postal system. Each member country agrees to the same set of terms for conducting international postal duties.
Prior to the UPU, a country would have to have a separate postal treaty for each country that it wished to carry international mail to or from.
The UPU also operates the system of International Reply Coupon.
The UPU is the oldest international organization and was created in 1874, under the name "General Postal Union," as a result of the Treaty of Berne signed on October 9 1874. In 1878 the name was changed to to "Universal Postal Union."
The UPU can be found online at http://www.upu.int/