Caoshu shi 草書勢

There are two books with the title Caoshu shi 草書勢 "The forces of the grass script", one written by Cui Yuan 崔瑗, and one by Suo Jing 索靖. On the latter, see Caoshu zhuang 草書狀.

Caoshu shi 草書勢 is an essay on calligraphy written during the Later Han period 後漢 (25-220 CE) by Cui Yuan 崔瑗 (77-142 CE), courtesy name Ziyu 子玉, who was administrator (xiang 相) of the Prince of Jibei 濟北. Cui was a disciple of the famous calligrapher Du Du 杜度 (mid-1st cent. CE). The essay is the oldest Chinese text on the cursive writing style called "grass script" (caoshu 草書). Cui advocates the use of the cursive grass script to make communication easier in the vast field of administration because the writings style was easier to use and the seal script (zhuanshu 篆書) or the chancery script (lishu 隸書). The grass script was more flexible and was not subject to the strict rules of the "old-style" scripts (gushi 古式), but instead attenuated the rigid principle of that "squared must be centered by a ruler, and circles must be accompanied by a pair of compasses" (fang bi zhong gui, yuan xu fu gui 方必中矩、圓須副規). Yet this did not mean that the grass script led to chaotic captions, because the caoshu-type script, too, had to pay attention to the subtlety of slender brush strokes, and to respect the right appropriate position in the context (xianwei yao miao, linshi cong yi 纖微要妙，臨事從宜).

Seen from the artistic aspect, Cui Yuan compares the appearance of grass-script texts with animals:

These appearances were the result of emotions and moods lasting during the writing process, a phenomenon not occurring by the use of seal or chancery script.