From the Sui and Tang dynasty, China was again unified after the period of disunity since the Han. The first half of the Tang was a culturally and economically rich period, but in the last part things gradually fell apart because of decadence at the court, military defeats and rebellions. 
The Kai Yuan Tong Bao was the first coin minted in the Tang dynasty. Kai Yuan meant “to open a new beginning”, and it did indeed set new standards for casting, legend, style and shape, that were followed in almost all later dynasties. The legend of Tang coins consisted of four characters read top-bottom-right-left. 
The two first characters on coins were from the 8th century onwards the emperors reign title. The two last characters on coins were from now on Tong Bao (circulating treasure), Zhong Bao (heavy treasure) or Yuan Bao (primary treasure) (Yang p. 24). Read more about legends. 

The standard weight of the Kai Yuan was one qian, but an interesting thing about Tang coins is, that for the first and only time in China's monetary history, the coins grew bigger and heavier during the dynasty (Ren p. 39).
Many coins of the Tang had a crescent, dot, sun, moon, star or cloud in different positions on the reverse. Peng writes that historians and numismatists agree that the first crescent was made by an empress, who marked the wax mould with her fingernail, but there seems to be nothing to prove the story (p. 252). 
The maximum annual output in Tang were according to von Glahn (p. 49) 327.000 strings.

About the casting on coins in the Tang see Casting of Coins.