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Hatto I (born c. 850, Swabia—died May 15, 913) was the archbishop of Mainz and counsellor to the German king Arnulf of Bavaria, the last East Frankish Carolingian emperor; as regent for Arnulf’s son Louis the Child (900–911), he governed the German kingdom for the last member of the East Frankish Carolingian dynasty. Hatto was elected abbot of Reichenau (888) and Ellwangen (889) and archbishop of Mainz (891). A trusted adviser of King Arnulf, he...