A period of time in Chinese porcelain history where no official emperor's reign names were inscribed on imperial porcelain vessels. This period encompasses the three reigns of Zhengtong, Jingtai and Tianshun. Various other names like blank period, empty period had been used in the past. The term "interregnum period" was first coined by the eminent porcelain scholar John Alexander Pope.It was a mystery as to why official imperial porcelain were not inscribed with reign names. This had made identification and authentication of imperial porcelains of this three reign period very difficult. Many theories were put forward by scholars trying to explain this mystery. However, until now, no conclusive proof had been agreed by scholars and researchers. In recent years, many researchers and scholars of porcelain ventured into the research of this mystery and many articles and thesis were published. In China, quite a number of books and articles were published in recent years however these had not been translated into English or other languages. More researchers and scholars came to agree that there were indeed official kiln imperial porcelain being produced during these three reign periods. Recent archaeological excavations from China and marine discoveries from shipwrecks around the world only yield porcelains from civilian kilns. Officially until today, no conclusive evidence of any official imperial kiln porcelain with official inscription of the emperor's reign period (also called year mark) were found.