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How long did Louis I rule as emperor?

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Illuminated manuscript portrait of Louis the Pious as a Christian Roman emperor, holding a staff and shield, with Latin text arranged around and over the figure. — Louis I Louis the Pious as a Christian Roman Emperor, portrait from *De laudibus sanctae crucis* by Rabanus Maurus.

Louis I, also known as Louis the Pious, ruled as emperor for 26 years. He succeeded his father, Charlemagne, in 814 and reigned until his death in 840.12