Shu 疏 was an old type of document submitted by functionaries to the central government, high functionaries or to the throne. The word has the meaning of "direct". It was introduced during the Han period 漢 (206 BCE-220 CE) with the name shangshu 上疏 and was used to explain complex issues. Shu-type memorials were used for a wide range of topics, from admonitions, discussions, suggestions, petitions and recommendations to apologies, declarations of retreat, or the confession of mistakes in office. Zhuge Liang's 諸葛亮 (181-234) Jieting zibian shu 街亭自貶疏, for instance, is a confession of his strategic mistakes and the proposal to accept a demotion in rank. Other famous early shu documents are Jia Yi's 賈誼 (200-168 BCE) Lun jizhu shu 論積貯疏 and Chen zhengshu shu 陳政事疏 or Chao Cuo's 晁錯 (d. 154 BCE) Lun guishu shu 論貴粟疏.
The use of shu-type memorials continued from the Tang 唐 (618-907) to the Qing period 清 (1644-1911), as can be seen in many surviving examples like Wang Zheng's 魏征 (580-643) Jian Taizong shi si shu 諫太宗十思疏, Zeng Gong's 曾鞏 (1019-1083) Yi Cangzhou guoque shangdian shu 移滄州過闕上殿疏 or Huang Juezi's 黃爵滋 (1793-1853) Yansai lou'e yi pei guoben shu 嚴塞漏厄以培國本疏, in which he urges to control strictly the illegal import of opium. From the Song period 宋 (960-1279) on, the difference between shu-type and zou 奏-type memorials blurred, and the term zoushu 疏奏 appeared, used as a general word for memorials.
Occasionally, shu-type documents were also used as letters between private persons, like Tao Yuanming's 陶淵明 (Tao Qian 陶潛, c. 365-427) Yu zi Yan deng shu 與子儼等疏.