A.E.I.O.U.

"A.E.I.O.U." was a symbolic device personally used by the Habsburg emperor Frederick III (1415–93), who had a fondness for mythical formulae. He habitually signed buildings such as Burg Wiener Neustadt or Graz Cathedral as well as his tableware and other objects with the acronym. Up to today, A.E.I.O.U. is the motto of the Theresian Military Academy established in 1751.

Frederick already had begun to use the signature when he was elected King of the Romans in 1440. He did not explain its meaning at the time, though, shortly before his death, he claimed it stood for German: Alles Erdreich ist Österreich untertan (MKL 1890) or "All the world is subject to Austria."

However other interpretations have been put forth even by contemporary heraldists. Several explanations proceed on the assumption that it was meant as a political slogan, from the Latin phrases:

More than 300 Latin and German interpretations have been attempted over the centuries; most of these versions refer to a motto of present-day Austria or the huge Habsburg Monarchy.

...
Wikipedia