Interregnum Period

The "Interregnum" period was a period in the Ming dynasty covering three reigns and two emperors.
The first and third emperor of that period was the same, with the reign name differing, the second reign was the emperor's brother, who usurped the throne.

This concerns the period from 1436 to 1464:

Zhengtong 1436 ~1449 
Jingtai 1450 ~1457
Tianshun 1457 ~1464

Short desciption of events
When Mongols attacked China the young emperor participated himself in the field and was captured. To avoid that the empire was without an emperor, the brother was made interim emperor. After a few years the captive emperor was released and returned to the capital. However, he was put under house arrest by his brother who usurped the throne. Only after he passed away was the actual emperor able to regain the throne.

Thus, the Zhengtong and Tianshun reigns, above, are different reign names of the same emperor, while Jingtai was the reign of his brother.