Kotaishiki indicates the laws concerns the replacement of a government officer with a new officer under the Ritsuryo system (a system of centralized government based on the ritsuryo code) in Japan, matters necessary for continuing the duties of the office and how they should be handed over to the successor. They were collected and compiled by Kageyushi, and the replacement procedure was conducted based on Kotaishiki.
As early as the eight century when the Ritsuryo system was enforced on a large scale, a privately compiled book called 'Kotaishiki' existed, in which the laws and regulations concerning the procedures for replacing government officers, in particular, kokushi (provincial governors) (Shochoku - imperial edicts) and Daijokanpu (official documents issued by Daijokan, Grand Council of State) were collected. However, privately compiled 'Kotaishiki' included many unsatisfactory items.
Around 797, kageyushi was introduced to audit results of the administration when a kokushi (provincial governor) was to be replaced. At that time, many problems concerned with the replacement of kokushi, such as "the procedures that were used were inadequate" and disputes between a predecessor and the successor, were caused, making it necessary to clarify the procedures for replacing a kokushi. Then in 803 SUGANO no Mamichi, a kageyushi, compiled "Sentei kotaishiki" (literally, selected kotaishiki) (Enryaku kotaishiki) (literally, kotaishiki in the Enryaku era), approved by Emperor Kanmu and was handled as the rules covering official replacement of kokushi.
In 824 and later, naikan (officers living in Kyoto) in addition to kokushi (officers living outside Kyoto) were also audited by kageyushi. Around 869, "Shinjo naigekan kotaishiki" (literally, new kotaishiki for officers living in Kyoto as well as those outside Kyoto) (Jogan kotaishiki) (literally, kotaishiki in the Jokan era), in which laws and regulations concerning replacement of officers living in Kyoto as well as those outside Kyoto were collected, were compiled by kageyushi. After this, when a government officer was replaced, the procedure of handing over matters necessary for continuing office duties were carried out based on this kotaishiki, regardless of whether he lived in or outside Kyoto.
In the first half of the tenth century when Emperor Daigo tried to reconstruct the organization of government according to the ritsuryo codes and started the work to compile Engi no kyakushiki (regulations and laws of the Engi era), kotaishiki was recompiled in the work as well. Compilation started in 911 and was completed in 921 as "Naigekan kotaishiki" (Engi kotaishiki) (literally, kotaishiki in the Jokan era). This became the last kotaishiki.
By the way, although it was called 'kotaishiki' (indicating a kind of 'shiki') from the time it was enforced for the first time, there is also the following opinion: According to the concept of 'kyakushiki' (kyaku indicates supplementary laws or regulations, and shiki indicates detailed rules for enforcing them) in the Ritsuryo laws, use of 'kyaku' is more appropriate.
Summary of each kotaishiki
Shinjo naigekan kotaishiki (Jogan kotaishiki)
Enforced in 868; Compiled by MINABUCHI no Toshina and others?; Two volumes, the upper volume and the lower volume.
Naigekan kotaishiki (Engi kotaishiki)
Established in 921; Complied by TACHIBANA no Kiyozumi and others; One volume including 192 articles.