This comes from two historical periods: "the flourishing age of Kaiyuan" and "the rule of Zhenguan":
The rule of Zhenguan refers to the peaceful and prosperous times in the early Tang Dynasty. Because Emperor Taizong can be virtuous, he knows people and makes good use of them. Open the way, modestly coachable, reuse Wei Zhi, etc. And adopted the agriculture-oriented, tax reduction, rest and recuperation, strict economy, improve the imperial examination system and other policies to stabilize the society. At that time, the national title was Zhenguan (627-649), which was called Zhenguan Governance in history. This was the first prosperous period of the Tang Dynasty, which laid the foundation for the prosperity of Kaiyuan later.
The flourishing period of Kaiyuan is the Kaiyuan period of Xuanzong of the Tang Dynasty (7 13 ~ 74 1), which is a prosperous period of the Tang Dynasty, with stable political situation, prosperous economy, prosperous culture and strong national strength. Historically, this period was called "the prosperous time of Kaiyuan".
Looking at Wu Zetian's administration, it can be said that it is a connecting link between the preceding and the following, and there is a saying that "politics opens a new era and governs macro festivals".