Werner Keller's Diaspora: The Post-Biblical History of the Jews (New York, 1969) describes the results of Pope Innocent II's Fourth Lateran Council of 1215. One of canons mandated that:
. . . Jews "whether men or women, must in all Christian countries distinguish themselves from the rest of the population in public places by a special kind of clothing." The reason given for this was to prevent "criminal" sexual intercourse between Christians and Jews. "So that," the text declared, "henceforth in case of such criminal intercourse no mistake can be alleged as an excuse." In November, 1215, a papal bull gave these decisions of the Council the force of canon law.
The dreadful Jewish badge had been created -- that dishonoring sign which for six centuries was to expose the Jews to public contempt everywhere they went in Europe.
The bull confined itself merely to ordering the distinguishing mark. The legislators of each country were to execute this decree as they saw fit. Princes and provincial councils henceforth discussed the type, shape, and color of the identifying mark. Two of Innocent's successors, Popes Gregory IX and Innocent IV, repeatedly reminded rulers to pay strict attention to the requirement and to allow no exceptions to the wearing of badges. Gradually, these "Cain's marks" became a common sight in all of Europe, their wearers identifiable everywhere at a distance.
England chose a badge depicting two stone tablets inscribed with the Ten Commandments. In France, St. Louis ordered a badge to be made of red felt or saffron-yellow cloth, cut in the shape of a wheel and worn on the upper garment, one in front and one in back, "so that those thus branded may be recognized from all sides." . . .
Germany instituted the rotella, a patch of yellow cloth in the shape of a wheel or an O. In some countries a simple badge was considered inadequate, and the wearing of a hat of a specified color was also prescribed. . . .
The marks of the badge and hat, however, were a prelude of the abuse and violence which were to follow:
. . . Other ordinances designed to humiliate Jews in public were devised. In some places it was regarded as the privilege of the rabble to pelt the Jews with stones at Easter; in other places, representatives of the Jewish community were made to accept blows or slaps in public at this season. . . .
The marking of Jews so that "those thus branded may be recognized from all sides" was just the beginning of what Werner Keller calls the "Medieval inferno."
During the Nazi regime, Jews were again forced to wear distinguishing marks -- most often a yellow cloth patch in the shape of a Star of David labeled "Jew" (e.g., Jude in German, Jood in Dutch, or Juif in French).
Keller, Werner. Diaspora: The Post-Biblical History of the Jews, trans. Richard and Clara Winston (New York: Harcourt, Brace, and World, Inc., 1969), pp. 213-214.