Kaiyuan Tongbao "is a currency minted on August 2, 621 AD, in the fourth year of Emperor Gaozu of Tang Dynasty's Wude reign. This is the longest and most important circulating currency of the Tang Dynasty. The top, bottom, right, and left characters on the coin face are read as' Kaiyuan Tongbao ', while the top, right, bottom, and left characters are read as' Kaiyuan Yuanbao'. Although "Kaiyuan" was a reign title of Emperor Xuanzong of Tang, "Kaiyuan Tongbao" was not a reign title coin, nor was it a currency that only began to be used during the reign of Emperor Xuanzong of Tang. The main reason why Tang Gaozu's reign title was named "Kaiyuan Tongbao" instead of "Wude Tongbao" was that there was no market for reign title coins at that time.