Yi 移, also called yiwen 移文, wenyi 移文, yishu 移書 or gongyi 公移, was a type of document used as a dispatch or letter addressed to equals or persons of inferior standing. It has the character of instruction and admonition, and is often used in combination with documents of the type xi 檄, leading to the expression xi-yi 檄移 or yi-xi 移檄. While the latter is directed to an adversary, yi are used to convince or encourage allies.
Yi and xi documents were used in pre- and early imperial times, for instance, in Sima Xiangru's 司馬相如 (179-117 BCE) Nan Shu fulao 難蜀父老. From the Wei period 曹魏 (220-265) on, yi documents were parts of military dispatches addressed to functionaries and commanders. The literary critique Wenxin diaolong 文心雕龍 (ch. Xi yi 檄移) explains with a word-pun that yi meant yi 易 "to change", i.e. to move a document to another institution.
Other exemplary yi documents are Wei Ao's 隗囂 (d. 33 CE) Yixi gao junguo wen 移檄告郡國文, Yu Pu's 虞溥 (c.238-300) Yigao shuxian 移告屬縣, Lu Ji's 陸機 (261-303) Yi baiguan 移百官 or Kong Zhigui's 孔稚珪 (447-501) Beishan yiwen 北山移文.
Yet yi-type documents were also used as dispatches between Central government institutions of the same level. During the Tang period 唐 (618-907), they were one of three types of lateral documents, the others being guan 關 "orders, instructions" and ci 刺 "announced communication". If sent to an institution of lower rank of the local administration (provinces, regions), yi-type documents were issued by one of the three departments (sansheng 三省).
For lateral documents between Central Government institutions, the administration of the Song dynasty 宋 (960-1279) used the term xingyi 行移, while the word die 牒 was used on the local level. The word yi was still used during the Qing period 清 (1644-1911) for dispatches between different institutions of the same level.
The word yi can also be used verbally, "to send a document".