The use of "Kaiyuan Tongbao" is also a copper coin
About the founding of "Kaiyuan Tongbao" "New Tang Book. Food and Huo Zhi" contains "In July of the fourth year of Wude, five bahts were discarded. The money is Kaiyuan Tongbao money, eight cents in diameter, weighing two baht and four taels, and weighing one tael for ten coins. The money is still in custody in Luo, Bing, You, Yi and other states. The King of Qi each gave three furnaces of coins, and the right servant She Pei Ji gave one furnace." This move marks the official withdrawal of China's currency named after two bahts from the stage of history, and will be replaced by a new "Baowen Qian" currency system.
(Note 2: The so-called "Baowen Qian" refers to the decimal currency system with one qian