The circuit (lu 路) was a territorial administrative unit used mainly during the Song period 宋 (960-1279). Circuits were directly responsible to the central government. The size of circuits roughly corresponded to the territories known as provinces today (like Fujian, Guangdong, Guangxi, Hunan, Hubei, Shaanxi or Heibei). Similar units had been created by the Tang 唐 (618-907), who called them dao 道. In the last third of the Tang period, special officials called transport commissioners (zhuanyunshi 轉運使) were appointed who were responsible for the correct remittances of taxes from each circuit to the capital. The "way" by which these funds were transported (shuilu 水路, lulu 陸路, i.e. riverine or land transport) is the origin of the term lu. In the course of time the responsibilities of the commissioners were expanded, and they supervised more civilian affairs, as well as judicial and censorial matters. The transport commissioners so became important regional officials exerting great influence.
At the beginning of the Song period, there were 21 circuits in the empire (in 979), which were reduced to 19 a few years later (982), then to 17 (989), and finally 16 (992). In 997 Emperor Taizong 宋太宗 (r. 976-997) decreed the fix number of 15 circuits. These were by Emperor Shenzong 宋神宗 (r. 1067-1085) split up to a number of 23 (plus the capital), namely:
|Kaifeng fu 開封府 (i.e. the capital city)|
|Jingdong-Dong 京東東路, Jingdong-Xi 京東西路|
|Jingxi-Nan 京西南路, Jingxi-Bei 京西北路|
|Hebei-Dong 河北東路, Hebei-Xi 河北西路|
|Yongxing jun 永興軍路 (a military prefecture)|
|Huainan-Dong 淮南東路, Huainan-Xi 淮南西路|
|Jiangnan-Dong 江南東路, Jiangnan-Xi 江南西路|
|Jinghu-Nan 荆湖南路, Jinghu-Bei 荆湖北路|
|Chengdu fu 成都府路|
|Guangnan-Dong 廣南東路, Guangnan-Xi 廣南西路|
With the loss of northern China to the Kitans 契丹 and then to the Jurchens 女真, the total number of circuits of the Song empire went down (the lost circuits were Kaifeng fu, Jingdong-Dong, Jingdong-Xi, Jingxi-Bei, Hebei-Dong, Hebei-Xi, Hedong, Qinfeng and Yongxing jun). The Southern Song empire 南宋 (1127-1279) was administrated in 16 circuits, the names of which were the same than before, except Liangzhe, which was split up into Liangzhe-Dong 兩浙東路 and Liangzhe- Xi 兩浙西路, and Zizhou, which was renamed Tongchuan fu 潼川府路.
While the circuits of the transport commissioners were general civilian administrative units, there were also special ones under the jurisdiction of regulatory commissioners (jinglüeshi 經略使) and pacification commissioners (anfushi 安撫使). They served as ad-hoc units when military campaigns were undertaken, and therefore not fix. Yet some regions, particularly in the north, had a permanent status of alert, like Qinfeng, Jingyuan 涇原, Huanqing 環慶, Luyan 鄜延, the military prefecture of Yongxing, and Xihe 熙河, as well as the prefectures of Daming 大名府, the Pass of Gaoyang 高陽關, Zhending 真定府 and Dingzhou 定州.
There was never a unified administration of circuits, but in order to prevent the rise of local potentates, the Song government had circuits administered by separate jurisdictions: the transport commissioners (zhuanyunsi, also called caoyunshi 漕運使) were responsible for finance and the recruitment of officials, the judicial commissioners (tixing 提刑, full designation tidian xingyu gongshi 提點刑獄公事) for law, the military commissioners for the garrisons, jails, execution of the penal law, monetary and granary policy, household registers and corvée labour. The highest official status of a commissioner in one circuit determined its character as a "transport circuit" (caosi lu 漕司路), a judicial circuit (xiansi lu 憲司路) or a military circuit (shuaisi lu 帥司路). The character of circuits tended to be more of the military type under the Southern Song.
The Jin empire 金 (1115-1234) of the Jurchens and the Yuan empire 元 (1279-1368) of the Mongols 蒙古 imitated the circuit system of the Song. The Jin empire was divided into 19 circuits of five different types, namely area command circuits (zongguanfu lu 總管府路), transport commission circuits (zhuanyunsi lu 轉運司路), surveillance commission circuits (anchasi lu 按察司路), army command circuits (tongjunsi lu 統軍司路), and circuits of bandit suppression commissions (zhaotaosi lu 招討司路). The Yuan "invented" the modern province (xingsheng 行省, later sheng 省), and reorganized the jurisdiction of the circuit, making it an intermediary territorial administration between the province and the prefecture. The Ming dynasty 明 (1368-1644) abolished the circuits.