How did calligraphy develop and progress in Sui and Tang Dynasties?

In the Sui Dynasty, there was a situation in which the styles of northern and southern calligraphy merged. Longzang Temple Monument and Dong Epitaph are the representatives. Scholars and calligraphers are mainly wise and brave monks, descendants of Wang Xizhi. In the calligraphy theory, people began to pay attention to the summary of writing techniques and laws, and the first article devoted to "Fa" appeared-a poem "Ode to the Heart" by the monk Zhiguo. The Tang Dynasty was the heyday of feudal society in China, and calligraphy also appeared the heyday characterized by "attaching importance to law". Four great calligraphers appeared in the early Tang Dynasty, such as Yu, Chu and Xue, and Li Yong, Yan Zhenqing and Liu Gongquan. In cursive script, there were grandchildren before, followed by Zhang Xu and Huai Su. Zhang Xu and Huai Su, in particular, created a brand-new Wild Grass on the basis of predecessors' cursive scripts, which reached a state of perfection and had a far-reaching impact on later generations. Seal script and official script have also developed and formed their own characteristics, but from the whole history of calligraphy, the achievements of seal script and official script in Tang Dynasty are not high and their influence is not great. Li is the representative of seal script, and Han Zemu, Cai, Shi and Liang Sheng Qinghe are the representatives of official script.