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How was Julian related to Constantine the Great?

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Julian Julian the Apostate, detail of a marble statue; in the Louvre, Paris.

Julian was the nephew of Constantine the Great. Specifically, Julian was the son of Julius Constantius, who was the half brother of Constantine I.1

Following the death of Constantine the Great in 337, the imperial succession passed to Constantine's sons, one of whom—Constantius II—was Julian's cousin.21