Though formerly government officials exchanged documents in a form of "I" (移) under the ritsuryo system, distinction became vague with an increase in number of Ryoge no kan, who used cho, and officials issued more and more documents in a form of cho instead of "I" (移).
以後も、律令制官司間のやりとりは移で行われていたが、牒を用いる令外官の増加とともに曖昧となっていき、次第に移に代わって牒の書式で作成されるようになっていった。