German postal reformer Heinrich von Stephan had suggested to introduce the "Postblatt," or open post sheet, as a cheap alternative to letters in 1865. But critics weren't ready to lose the privacy of their correspondence. Similar cards had already been sent in the US and the UK, but the "correspondence card" format was officially introduced by the Austro-Hungarians on October 1, 1869.

Despite the critics' worries, the German public enthusiastically adopted the postcard. On the day they were introduced in Berlin, on June 25, 1870, over 45,000 copies were sold. In 1885, images were officially allowed on postcards. This postcard from 1900 depicts how the new medium was all the rage: Nearly a billion postcards were sent from the German Empire that year.

This postcard from 1905 shows a postboy delivering mail to a post box, accompanied by his delivery bike. Not only lithographic drawings served as templates for postcards, but also photographs. The standardized format also incited people to collect them in albums — especially when they came from an admirer, such as this card, written to a woman with whom the author spent "hours in love."

This card was sent from Egypt's Port Said in 1899. A week later, traveling by steamboat and by train, it had already reached its destination, the northern German city of Schwerin. The first images on postcards were designed to leave space for the message. In 1905, the address side of the card was divided in two: From then on, the message was written on the left side and the address on the right.

Remember how the media spread the Millennium Bug hysteria as we approached the year 2000? Instead of such anxiety, this postcard from 1900 celebrated with quasi-surreal optimism the turn of the century: Shown in the background of this sunny new age is a telegraph line, a train, a steamboat and smoking factory chimneys, while the young prophetic character holds a palm branch, a symbol of peace.

As we however know, it wouldn't be a century of peace. In 1898, it was a privilege to watch a military parade led by Kaiser Wilhelm II, and that was something to write home about. The author of this card mentions that he even had the chance to meet the emperor himself.

This picture postcard from 1906 shows the happy reactions of soldiers as they receive a package or a letter. It also provides a form to quickly fill out news on one's heath / hunger / thirst / wallet. Postcards were already extremely popular during the Franco-German War in 1870-71, and soldiers sent some 10 billion field postcards during the First World War — all free of charge.

Just like we might share photos of tragedies we've witnessed or that affect us on social media today, picture postcards of dramatic events were also quickly printed to be sent out to the world. This one shows the site of the accident of a "terrible catastrophe on an elevated railway in Berlin on September 26, 1908."

Wilhelm Voigt was a conman who dressed up as a Prussian military officer and convinced a group of soldiers to follow his command and rob a municipal treasury in Köpenick. Even the kaiser found his caper so impressive that he pardoned him before the end of his prison sentence. "The Captain of Köpenick" became a folk hero through plays, a figure in the wax museum — and postcards such as this one.

Austrian artist Raphael Kirchner was renowned for his Art Nouveau works, such as this one from 1904. But with his depictions of women in erotic poses, he was also an influential painter in the pin-up genre. European and American soldiers would collect such postcards during World War I. During World War II, people increasingly preferred to send their news in sealed letters instead of postcards.