"A.E.I.O.U." was a symbolic device coined by Emperor Frederick III (1415–1493) and historically used as a motto by the Habsburgs. One note in his notebook, though not in the same hand, explains it in German and Latin as "All the world is subject to Austria". Frederick habitually signed buildings such as Santa Maria dell'Anima in Rome, Burg Wiener Neustadt, or Graz Cathedral as well as his tableware and other objects with the vowel graphemes. A.E.I.O.U. is also the motto of the Theresian Military Academy, established in 1751. It can also be found on the wall of the Chancellor's office in the Federal Chancellery of Austria. The famous device is probably the most known motto of premodern times, because it has repeatedly been given new interpretations. Unraveling the mystery of what the AEIOU means is part of a centuries-long debate that is still ongoing today.

Frederick III was Holy Roman Emperor from 1452 until his death in 1493. He was the penultimate emperor to be crowned by the pope, and the last to be crowned in Rome.