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=== Legalities and censorship ===
The initial appearance of picture postcards (and the enthusiasm with which the new medium was embraced) raised some legal issues. Picture postcards allowed and encouraged many individuals to send images across national borders, and the legal availability of a postcard image in one country did not guarantee that the card would be considered "proper" in the destination country, or in the intermediate countries that the card would have to pass through. Some countries might refuse to handle postcards containing sexual references (in seaside postcards) or images of full or partial nudity (for instance, in images of classical statuary or paintings). For example, the United States Postal Service would only allow the delivery of postcards showing a back view of naked men from Britain if their posteriors were covered with a black bar.<ref>{{cite news |date=30 July 2001|title=Naked film postcards returned to sender |work=BBC News |url=http://news.bbc.co.uk/2/hi/uk_news/wales/1464627.stm |access-date=2011-06-05}}</ref> Early postcards often showcased photography of nude women. Illegal to produce in the United States, these were commonly known as [[French postcard]]s, due to the large number of them produced in France. Other countries objected to the inappropriate use of religious imagery. The [[Ottoman Empire]] banned the sale or importation of some materials relating to the [[Prophets in Islam|Islamic prophet]] [[Muhammad]] in 1900. Affected postcards that were successfully sent through the Ottoman Empire before this date (and are postmarked accordingly) have a high rarity value and are considered valuable by collectors.

==== Lynchings ====
{{Main article|Lynching postcards}}

In 1873, the [[Comstock Act]] was passed in the United States, which banned the publication of "obscene matter as well as its circulation in the mails".<ref name=":3">Kim, Linda (2012). "A Law of Unintended Consequences: United States Postal Censorship of Lynching Photographs". ''[[Visual Resources]]''. Taylor & Francis. '''28''' (2): 171–193. [[Digital object identifier|doi]]:10.1080/01973762.2012.678812.</ref> In 1908, §3893 was added to the Comstock Act, stating that the ban included material "tending to incite arson, murder, or assassination".<ref name=":3" /> Although this act did not explicitly ban lynching photographs or postcards, it banned the explicit racist texts and poems inscribed on certain prints. According to some, these texts were deemed "more incriminating" and caused their removal from the mail instead of the photograph itself because the text made "too explicit what was always implicit in lynchings".<ref name=":3" /> Some towns imposed "[[self-censorship]]" on lynching photographs, but section 3893 was the first step towards a national [[censorship]].<ref name=":3" /> Despite the amendment, the distribution of lynching photographs and postcards continued. Though they were not sold openly, the censorship was bypassed when people sent the material in [[envelope]]s or mail wrappers.<ref>Apel, Dora (2004). ''Imagery of Lynching: Black Men, White Women, and the Mob''. New Brunswick, N.J.; London: [[Rutgers University Press]]. {{ISBN|978-0-8135-3459-6}}.</ref>

==== World War I ====
{{Main article|Postal censorship}}

Censorship played an important role in the First World War.<ref name=":16">{{Cite web|url=https://encyclopedia.1914-1918-online.net/article/censorship|title=Censorship {{!}} International Encyclopedia of the First World War (WW1)|last=Demm|first=Eberhard|website=International Encyclopedia of the First World War|url-status=live|archive-url=https://web.archive.org/web/20200120151713/https://encyclopedia.1914-1918-online.net/article/censorship|archive-date=2020-01-20|access-date=2020-04-03}}</ref> Each country involved utilized some form of censorship. This was a way to sustain an atmosphere of ignorance and give propaganda a chance to succeed.<ref name=":16" /> In response to the war, the United States Congress passed the [[Espionage Act of 1917]] and [[Sedition Act of 1918]]. These gave broad powers to the government to censor the press through the use of fines, and later any criticism of the government, army, or sale of war bonds.<ref name=":16" /> The Espionage Act laid the groundwork for the establishment of a Central Censorship Board which oversaw censorship of communications including cable and mail.<ref name=":16" />

Postal control was eventually introduced in all of the armies, to find the disclosure of military secrets and test the morale of soldiers.<ref name=":16" /> In [[Allies of World War II|Allied countries]], civilians were also subjected to censorship.<ref name=":16" /> French censorship was modest and more targeted compared to the sweeping efforts made by the British and Americans.<ref name=":16" /> In Great Britain, all mail was sent to censorship offices in London or Liverpool.<ref name=":16" /> The United States sent mail to several centralized post offices as directed by the Central Censorship Board.<ref name=":16" /> American censors would only open mail related to Spain, Latin America or Asia—as their British allies were handling other countries.<ref name=":16" /> In one week alone, the San Antonio post office processed more than 75,000 letters, of which they controlled 77 percent (and held 20 percent for the following week).<ref name=":16" />

Soldiers on the front developed strategies to circumvent censors.<ref name=":17">{{Cite web|url=https://ww1.habsburger.net/en/chapters/circumventing-censorship-and-self-censorship|title=Circumventing the censorship and "self-censorship"|website=The World of the Habsburgs|date=11 June 2014 |url-status=live|archive-url=https://web.archive.org/web/20181221102438/https://ww1.habsburger.net/en/chapters/circumventing-censorship-and-self-censorship|archive-date=2018-12-21|access-date=2020-04-02}}</ref> Some would go on "home leave" and take messages with them to post from a remote location.<ref name=":17" /> Those writing postcards in the field knew they were being censored, and deliberately held back controversial content and personal matters.<ref name=":17" /> Those writing home had a few options including free, government-issued field postcards, cheap, picture postcards, and embroidered cards meant as keepsakes.<ref name=":18">{{Cite web|url=https://encyclopedia.1914-1918-online.net/article/war_letters_communication_between_front_and_home_front#Postcards_2C_Parcels_2C_and_Family_Correspondence|title=War Letters: Communication between Front and Home Front {{!}} International Encyclopedia of the First World War (WW1)|last=Hanna|first=Martha|date=2014-10-08|website=International Encyclopedia of the First World War|url-status=live|archive-url=https://web.archive.org/web/20190326175924/https://encyclopedia.1914-1918-online.net/article/war_letters_communication_between_front_and_home_front|archive-date=2019-03-26|access-date=2020-04-03}}</ref> Unfortunately, censors often disapproved of picture postcards.<ref name=":18" /> In one case, French censors reviewed 23,000 letters and destroyed only 156 (although 149 of those were illustrated postcards).<ref name=":18" /> Censors in all warring countries also filtered out propaganda that disparaged the enemy or approved of atrocities.<ref name=":16" /> For example, German censors prevented postcards with hostile slogans such as "{{Lang|de|Jeder Stoß ein Franzos}}" ("Every hit a Frenchman") among others.<ref name=":16" />