Excerpts from Books and Wikipedia
"His extraordinary decision to hand over the crown of Spain to his son, Philip II, in January 1556 directly followed his resignation of the Netherlands and the Duchy of Burgundy to Philip. . . Unhappy with events in Germany, he had placed the running of the Holy Roman Empire in the hands of his brother Ferdinand after the religious compromise with the Protestant princes at Augsburg in 1555. . . . and from now on the Habsburg dynasty would remain split between its Austrian and Spanish branches. Nine members of the Austrian Hapsburgs were elected Holy Roman Emperor from 1558 until 1740." [Furtado: 1001 Days]