Editing Universal Postal Union (section)

=== Further developments ===
The Treaty of Bern had been signed by 21 countries, 19 of which were European.{{NoteTag|The Austrian and Hungarian delegates signed separately, but the preamble to the treaty considered [[Austria-Hungary]] to be a single country.}} After the General Postal Union was established, its membership grew rapidly as other countries joined. At the second [[Postal Union Congress]] in 1878, it was renamed the Universal Postal Union.<ref name="upu_history" />

French was the sole official language of the UPU until English was added as a working language in 1994. The majority of the UPU's documents and publications—including its flagship magazine, ''Union Postale''—are available in the United Nations' [[official languages of the United Nations|six official languages]]: French, English, Arabic, Chinese, Russian, and Spanish.<ref>{{cite web |title=Languages |url=https://www.upu.int/en/Universal-Postal-Union/About-UPU/History/Languages |publisher=Universal Postal Union |access-date=26 August 2020 |archive-date=26 May 2023 |archive-url=https://web.archive.org/web/20230526211326/https://www.upu.int/en/Universal-Postal-Union/About-UPU/History/Languages |url-status=live }}</ref>

Toward the end of the 19th century, the UPU issued rules concerning stamp design, intended to ensure maximum efficiency in handling international mail. One rule specified that stamp values be given in numerals, as denominations written out in letters were not universally comprehensible.<ref name="Johl">{{cite book |title=The United States Postage Stamps of the Twentieth Century, Volume I |year=1937 |publisher=H. L. Lindquist |first1=Beverly |last1=King |last2=Johl |first2=Max |page=104 }}</ref> Another required member nations to use the same colors on their stamps issued for post cards (green), normal letters (red) and international mail (blue), a system that remained in use for several decades.<ref>{{cite web |url=http://www.kenmorestamp.com/us-stamps/1890-1899-219-293/1898-universal-postal-union-colors-279-284 |title=1898 Universal Postal Union Colors #279-284 |publisher=Kenmore Stamp Company |access-date=28 September 2013 |archive-url=https://web.archive.org/web/20131102073441/http://www.kenmorestamp.com/us-stamps/1890-1899-219-293/1898-universal-postal-union-colors-279-284 |archive-date=2 November 2013 |url-status=dead }}</ref>
[[File:Universal Postal Union stamp 10c 1974 issue.jpg|thumb|upright|100 years of UPU commemorated on a US postage stamp]]

After the foundation of the United Nations, the UPU became a [[List of specialized agencies of the United Nations|specialized agency]] of the UN in 1948.<ref>{{cite web |title=About UN Specialized Agency |url=http://www.upu.int/en/the-upu/un-specialized-agency/about-un-specialized-agency.html |publisher=Universal Postal Union |access-date=9 October 2011 |archive-date=15 October 2011 |archive-url=https://web.archive.org/web/20111015042725/http://www.upu.int/en/the-upu/un-specialized-agency/about-un-specialized-agency.html |url-status=dead }}</ref> It is currently the third oldest intergovernmental organization after the [[Central Commission for Navigation on the Rhine|Rhine Commission]] and the [[International Telecommunication Union]].