Sangen (three major checkpoints) (三関)

Sangen (or Sankan) is a general term for the three important sekisho (a checkpoint) out of which were established in and around Kinai (five capital provinces surrounding the ancient capitals of Nara and Kyoto) in ancient Japan. It was also referred to Sangoku no Seki (seki of three provinces [seki is a short form of sekisho]). Originally it referred to the three checkpoints of Fuwanoseki (Mino Province), Suzukanoseki (Ise Province) and Arachinoseki (Echizen Province), but in the early ninth century Arachinoseki was replaced by Osakanoseki (Omi Province). And the ritsuryo-code-based provinces which include Sangen were called Sangen provinces.

Roles of Sangen
Sangen were always provided with some military instruments and weapons, and there was a rule that at least two kokushi (a provincial officer) of shitokan (four classifications of bureaucrats' ranks) should always be stationed to defend the seki. This shitokan officer stationed was called a castellan and prepared for emergencies.

When a state of emergency occurred, Chotei (Imperial Court) dispatched Kogenshi (a closer of a checkpoint who was dispatched in case of emergency) to Sangen. On this occasion Kogenshi was given a left half of a tally called Kankei (a tally used by military forces when they passed through Sangen in case of emergency) which was kept by Kurazukasa (a public office dealing with tallies for Sangen, clothes of Emperor and Empress, and so on) and compared it at Sangen with a right half which had been kept by Kokushu (the governor of a province). When the both tallied, the state of emergency was confirmed and the gate was closed. This is called Kogen. Kaigen (to open a gate of seki) after the state of emergency was solved took the same procedure as this.

By the way there was a view in the past that the gate was closed to prevent people from entering Kinai from Togoku (eastern provinces), but recently another view has been plausible that it was closed to prevent the state of emergency which had occurred in the capital from spreading to Togoku.
The reasons are mentioned as follows:
All the cases in which Kogen was put into practice were emergencies in the capital; all the Kokufu (an ancient provincial office) of the Sangen provinces was positioned at the east side of each seki, namely, outside of seki when it was seen from the capital; and so on. Also there was a purpose to prevent rebels against the capital from fleeing to Togoku; in FUJIWARA no Nakamaro's War in 764, closing Arachinoseki prevented Nakamaro from escaping to Echizen Province where his son lived.

History
Sangen is considered to be established in 672 or 673. It is known that in the Jinshin War, which happened around that time, Emperor Tenmu closed Fuwanomichi (an important gateway to Togoku, present Sekigahara of Gifu Prefecture) at the initial stage and got an advantage. After this, by the Taihoryo (Taiho Code) of the early eighth century, Sangen was legally designated to have both police and military functions. During the Wado era, due to an imperial command, two guards (military officers) were deployed for Kokushu of the Sangen provinces.

At the death of the Retired Emperor Genmei on January 2, 722, Kogen was performed for the first time. Thereafter, kogenshi was dispatched in cases of death or disease of emperors and retired emperors, and some battles such as Nagayao no Hen (Conspiracy of Nagayao), FUJIWARA no Nakamaro's War, the War of Kusuko, and so on.

On August 13, 789, by the imperial decree of Emperor Kanmu, Sangen of Fuwanoseki, Suzukanoseki and Arachinoseki was suddenly abolished. They were ordered to move weapons to Kokufu and relocate buildings and others to appropriate places. However, Sangen was not completely abolished and remained in operation.

At the times of Emperor Kanmu's death in 806 and the scheme to transfer the capital to Nara by the Retired Emperor Heizei in 810, an imperial edict to preserve Sangen was issued. In the former case, Osakanoseki, instead of Aracinoseki, was included in Sangen. Thereafter, Kogen of 840, 871, 884, and so on, has been recorded in "Rikkokushi" (Japan's six national histories chronicling the seventh and eighth centuries). Since then, in case of death of emperors and Sessho Kanpaku (regent and chief adviser to the Emperor), Kogen took place as designated in "Jogan Gishiki" (ceremonies in the manner of the Jogan era) and "Saikyuki" (a record of court practices and usage, written by MINAMOTO no Takaaki in Chinese style). Especially Kogen after the late tenth century became ceremonial and continued to the early-modern Tenpo era.