On May 29, 1942, German authorities issued a decree requiring all Jews in occupied France wear a yellow badge with a star. The requirement took effect on June 7, 1942.
But the Nazi's did not invent the practice of making Jews wear special markings.
The practice began in Muslim-dominated countries beginning in the early 8th century and continued for centuries.
According to The History of the Jews by Paul Johnson, a document from 1121 gave the following description of decrees issued in Baghdad:
Two yellow badges are to be displayed, one on the headgear and one on the neck. Furthermore, each Jew must hang round his neck a piece of lead with the word Dhimmi on it. He also has to wear a belt round his waist. The women have to wear one red and one black shoe and have a small bell on their necks or shoes.
The practice was adopted by the Catholic Church and continued well into the 18th century.
In 2014 the practice was revived by ISIL after the fall of Mosul in Iraq. ISIL demanded Assyrian Christians in the city to convert to Islam, pay tribute, or face execution. ISIL begun marking homes of Christian residents with the Arabic letter nūn for Nassarah ("Christian"). Thousands of Christians, Yazidis and others abandoned their homes and land.
In response to the persecution of Christians and Yazidis by ISIL, an international social media campaign was launched to raise global awareness of the plight of religious minorities in Mosul, making use of the letter ن (nun)—the mark that ISIL troops spray painted on deserted Christian properties.
Use this link to learn more about the Yellow Badge.
Use this link to learn more about the modern day use of the Arabic letter ن.