"Kaiyuan" has the meaning of creation and initiative, and "Bao Tong" is a treasure in circulation. There has been a great breakthrough in the unit of weight. In ancient times, the weighing method of 24 baht was one or two. Kaiyuan Bao Tong initiated the decimal system. Each piece weighs two baht, four pieces are one penny, and the accumulated ten pence weighs one or two, that is, ten pence is one or two "to replace baht with money".
Kaiyuan Bao Tong was cast in the Tang Dynasty for more than 200 years, which made the monetary system stable for a long time. The monetary system in which money was the unit of weight in the Tang Dynasty lasted for thousands of years in the Qing Dynasty. According to legend, the Kaiyuan Bao Tong cast in Tang Gaozu was praised as a calligraphy coin by Ouyang Jian, a calligrapher in the early Tang Dynasty.
After the Anshi Rebellion, Tang Suzong made a lot of money to cope with financial difficulties, which was called "dry yuan and heavy treasure". This is the earliest money called "Bao", and one treasure is the first ten pence, which has caused devaluation, soaring prices, serious counterfeiting and people's anxiety. At the end of the Tang Dynasty, Tang Wuzong abolished Buddhism and took a lot of money from Buddhist copper to cast "Huichang Kaiyuan", which eased the deflation that lasted for half a century. "Gan Yuan" and "Huichang" are both Gregorian years.
The Five Dynasties and Ten Kingdoms was a period of separatist disputes among warlords, and it was also a period of currency chaos, because there were many regimes and various currencies. Besides copper, there are also tin and iron. Large coins cost ten to one hundred, even thousands to ten thousand. The confusion and complexity of the monetary system reflects the economic deterioration caused by the separatist war.
To sum up, the square hole coin of copper coins has changed from the two systems of Thai baht to the year number, and the Bowen system is a major turning point in the development of money.