'Circulating treasure from the inauguration of a new epoch'), sometimes romanised as Kai Yuan Tong Bao or using the archaic Wade-Giles spelling K'ai Yuan T'ung Pao,[3] was a Tang dynasty cash coin that was produced from 621 under the reign of Emperor Gaozu and remained in production for most of the Tang dynasty until 907.

[9] The discovery of this clay mould has made it unclear as to what process was actually used to cast the Kaiyuan Tongbao cash coins.

[9] Under the Tang dynasty the earlier Wu Zhu coins of the Sui dynasty would remain the standard currency, but during the fourth year of the Wu De (武德) period (or 621 of the Gregorian calendar) Emperor Gaozu decreed that the Kaiyuan Tongbao coin be cast with a strictly enforced standard weight of 1⁄10 Liǎng (兩).

[14] The government of the Tang dynasty initially set up the Money Casting Bureau, which operated mints in a total of 14 locations.

[15][16] The reason that the Kaiyuan Tongbao also inspired the yuán bǎo legend is because the Chinese people themselves had trouble figuring out the correct character order, as the inscription is read in what was referred to as the "standard order" (top-bottom-right-left) some people accidentally read it in the wrong order as they had assumed that the inscription was read clockwise as Kaitong Yuanbao (開通元寶), this was also because rather than having the first two characters spell out the period title (which was Wu De when the Kaiyuan Tongbao was introduced), they had a different inscription.

[17][18][19][20] While the term tōng bǎo (通寶) was even used longer with the last Chinese cash coin, the Minguo Tongbao (民國通寶) being produced in Dongchuan, Yunnan during the early Republic of China period.

[15] The Emperor asked one of China's most well-known calligraphers, Ouyang Xun to write down the legend of the cash coin.

[14] At first, mints were set up in Luoyang in Henan, and also in Peking, Chengdu, Bingzhou (Taiyuan in Shanxi), and then Guilin in Guangxi.

[15] The Kaiyuan Tongbao cash coins that were first cast until the height of the Tang period, early issues can be very accurately assigned to their time of casting and archeological evidence from Tang era tombs indeed prove that the first stroke of the character "元" are shorter than later versions, for this reason these coins are referred to as "short one yuan" (短一元, duǎn yī yuán) versions.

[15] Later variants of the Kaiyuan Tongbao often have excess metal between the strokes of the Hanzi characters and even later variants have characters with strokes so long that they touch the rim, meanwhile the rims on the reverse side of these Kaiyuan Tongbao coins tend to be irregular and relatively flat.

[15][25][26] Huichang Kaiyuan Tongbao (simplified Chinese: 会昌開元通宝; traditional Chinese: 會昌開元通寶; pinyin: huìchāng kāiyuán tōng bǎo) cash coins are a series of Kaiyuan Tongbao coins produced under Emperor Wuzong who was a devout Taoist and used the reign era name of huìchāng (會昌), during the 5th year of this epoch (845) Emperor Wuzong ordered the casting of new coins with the inscription Kaiyuan Tongbao to be manufactured of bronze acquired by melting confiscated statues, copper bells, gongs, incense burners, and other copper items from Buddhist temples.

[5][27] The following mint marks could be found on Huichang Kaiyuan Tongbao cash coins:[15] 13 Kaiyuan Tongbao turtle shell coins (traditional Chinese: 玳瑁幣; simplified Chinese: 玳瑁币; pinyin: Dàimào bì), made from Hawksbill sea turtle shell, were discovered at the Famen Temple in 1987.

[28] Likely by the order of a Tang dynasty emperor to honour a sacred relic of Gautama Buddha that was located at the Famen Temple.

This tantra states that there are seven "treasures" or precious things: (1) gold, (2) silver, (3) pearl, (4) coral, (5) turtle shell, (6) crystal, (7) colored glaze."

As no mention of these turtle shell Kaiyuan Tongbao cash coins exist in any historical records or text both Chinese archaeologists and numismatists were surprised with the find.

'Bad money') were rampant during the Tang dynasty period, counterfeit Kaiyuan Tongbao cash coins were of inferior quality, they were typically lighter or were made with alloys containing larger percentages of cheaper metals, such as iron and lead, reducing the Intrinsic value of the cash coins in circulation.

[14] The introduction and circulation of counterfeit cash coins negatively affected the economy by causing inflation and reducing social stability.

[14] These factors all created a market incentive to produce counterfeit cash coins to fill the demand for currency.

[14] Researcher Liu of the Chinese numismatic society believed that the government regulations requiring high copper content in the official alloys were only introduced to curb the cash coin counterfeiting.

[14] Japanese "Fuhonsen" and later the Wadōkaichin were modelled after the Tang dynasty's Kaiyuan Tongbao coin using similar calligraphy.