guan 關, communication between institutions of the same level of local administration

Guan 關 was a type of document used for communication by functionaries of the lower levels of local administration, for use among officials of the same rank. It was introduced during the very late Eastern Han period 東漢 (25-220 CE) for communication between heads of local administration units. During the Ming period 明 (1368-1644), it was typically used for official letters exchanged between guard military commands (wei zhihuishi si 衛指揮使司), between prefects and between district magistrates. The Qing dynasty 清 (1644-1911) restricted the use of guan-type documents for civilian officials between the ranks of provincial administration commissioner (buzhengshi布政使) and district magistrate, and regional vice commander (fujiang 副將) and brigade vice commander (dusi 都司, see Green Standard Army) in the military sphere.