Incest of unequal gravity: the "incestus iuris gentium," which was applied to both Romans and non-Romans in the Empire, and the "incestus iuris civilis," which concerned only Roman citizens. Therefore, for example, an Egyptian could marry an aunt, but a Roman could not. Despite the act of incest being unacceptable within the Roman Empire, Roman Emperor Caligula is rumored to have had sexual relationships with all three of his sisters (Julia Livilla, Drusilla, and Agrippina the Younger). Emperor Claudius, after executing his previous wife, married his brother's daughter Agrippina the Younger, and changed the law to allow an otherwise illegal union.
Rudolf II, Holy Roman Emperor on 18 July 1552. He was the eldest son and successor of Maximilian II, Holy Roman Emperor, King of Bohemia, and King of Hungary and Croatia; his mother was Maria of Spain, a daughter of Charles V and Isabella of Portugal. He was the elder brother of Matthias who was to succeed him as king of Bohemia and Holy Roman Emperor. Rudolf spent eight formative years, from age 11 to 19 (1563–1571), in Spain, at the court of his maternal uncle Philip II. After his return to Vienna, his father was concerned about Rudolf's aloof and stiff manner, typical of