Frederick V of Austria

Frederick was elected King of the Romans in Frankfurt, succeeding his cousin Albert II. This election placed him at the head of the Holy Roman Empire, beginning his long reign as Frederick III.

Frederick III adopted the cryptic motto AEIOU, which he used on his personal belongings and buildings. The meaning remains debated, but it symbolized his claim to universal rule and Habsburg destiny.

Frederick III was crowned Holy Roman Emperor by Pope Nicholas V in Rome. This was the last imperial coronation to take place in Rome, and it solidified his authority over the empire.

Frederick III arranged the marriage of his son Maximilian to Mary of Burgundy, heiress of the Burgundian state. This union brought the Burgundian Netherlands into the Habsburg domain, greatly expanding Habsburg power.

Frederick III lost Vienna to the Hungarian king Matthias Corvinus after a prolonged siege. The city remained under Hungarian control until 1490, marking a significant territorial loss for Frederick.