Roman Empire
For Roman emperors the succession would often be an adopted son. The Emperor would notice an outstanding young man from one of the best families and would adopt him as his son. Before he died, he would make clear who he thought should succeed him. He could make his adopted son a Roman consul, or state in his will that the younger man should succeed him. Sometimes, that worked well. but not always. Every now and then, a civil war would be fought between claimants to the throne.