Duomo of Monza

In 817 AD, he had been declared heir and joint emperor of the Frankish realms with his father's Ordinatio Imperii, and in 823 AD, he was crowned as King of Italy with the Iron Crown.

The Iron Crown of Lombardy, now housed in Monza Cathedral in Italy, is an ancient symbol of power on the peninsula, and legend has it that Saint Helena, Constantine's mother, had it wrought around a nail from the True Cross, but its current parts are from different periods, some of which might date to the time of Constantine.

It had been used to crown Charlemagne as King of Lombardy back in 774 and is one of the great treasures of the medieval world passed down to us.

So Lothair was a powerful figure in the court and looked every inch the heir apparent to the entire empire after his father died or possibly retired to a monastery, which is not something out of the realms of possibility considering the amount of time Louis was spending in prayer.