Xishu 璽書 "sealed letters" (also called cishu 賜書 "granted letters") was a kind of edict an early imperial China, and thus a subtype of edicts and orders (zhaoling 詔令). The word xi was reserved for a jade seal the use of which was reserved to the sovereign. Only sealed imperial decrees were called xishu, regardless of what particular type the decree was. Wu Ne 吳訥 (1372-1457) says in his literary theory Wenzhang bianti xushuo 文章辨體 that the early Han-period 漢 (206 BCE-220 CE) emperors used three seals that were applied on some decrees. However, seals were not directly applied to the document material (bamboo, wood, fabric), but outside on a container on a separate seal paste (ni 泥) which held together the document. The first xishu letter is mentioned for the year 544 BCE (Zuozhuan 左傳, ch. Xianggong 襄公 29), but it was written and sealed not by the Duke of Lu 魯, but by his minister Ji Wuzi 季武子 (d. 535 BCE), which shows that only from imperial times on, xi-type seals were reserved to the emperor.
Xu Shizeng’s 徐師曾 (1517-1571) Wenti mingbian 文體明辨 explains that sealed letters were used for decrees (gaoyu 告諭), to reward persons (jianglao 獎勞), or for critique (zerang 責讓). The language style of sealed letters was usually praise and admonition in very modest and sincere words, and the text began thus with the words "The sovereign respectfully addresses X" (huangdi jing xiang mou guan mou 帝敬向某官某), or – more impersonal – "letter decreed to X" (shu yu mou guan 書諭某官).
Very famous is the sealed letter posthumously issued by the First Emperor to his heir, Prince Fusu 扶蘇 (d. 210), with the order to commit suicide in order to cede the throne to Prince Huhai 胡亥 (r. 209-207). Emperor Wen 漢文帝 (r. 180-157 BCE) of the Han dynasty issued the Xishu ci qian Chao Cuo 璽書賜簽晁錯, Emperor Zhao 漢昭帝 (r. 87-74 BCE) the Ci Yan ci Wang Dan xishu 賜燕刺王旦璽書, Emperor Yuan 漢元帝 (r. 49-33 BCE) the Xishu lao Feng Fengshi qie rang zhi 璽書勞馮奉世且讓之 and Emperor Guangwu 漢光武帝 (r. 25-57 CE) the Ci Dou Rong xishu 賜竇融璽書.
The use of the word xishu ended with the Tang period, but this does not mean that the use of (sealed) letters (shu 書) of the emperor sent to subordinates ended thereafter.