天啓帝
From the perspective of pedigree, Taichang Emperor had only two sons: Tianqi Emperor and Chongzhen Emperor. Tianqi Emperor was sonless, and the 3 sons of Chongzhen Emperor were missing. The successor can be only choose from uncles of Chongzhen Emperor, which were brothers of Taichang Emperor and other sons of Wanli Emperor. Zhu Changxun (Prince of Fu) was Wanli's 3rd son, Zhu Changhao (Prince of Rui) was 5th son, Zhu Changrun (Prince of Hui) was 6th son and the youngest was Zhu Changying (Prince of Gui, Zhu Youlang's father). Among these imperial uncles, Zhu Changxun was the eldest uncle, and Zhu Yousong was the eldest son of Zhu Changxun. As the imperial sons were missing, Zhu Yousong became the first in line to the succession.