Kani Sotosei (官位相当制)
Kani Sotosei was the bureaucrats hierarchy system that set the constant suitability relationship between Ikai (Court rank) and government post given to a government official on the Japanese Ritsuryo system (a system of centralized government based on the ritsuryo code).
Japanese ritsuryo official rank system was prescribed for Taiho-ryo (Taiho Code) and Yoro-ryo (Yoro Code) based on the Court Ranking Law, but its model was To-Kanhin-ryo (Kanhin Code of Tang [China]). Both set the correspondence relationship between Ikai (called Hinkai in Tang) and government post, but there was a big difference on its foundational rule.
On the Ritsuryo system of Tang, it was the common principle that hierarchy of government officials were decided by the government post, and Ikai was only the index to describe the grade of the government post. Government officials were supposed to receive the government post and Hinkai correspond to it, but the actual government posts had limitations. Therefore, some officials received nominal government post, and many officials became Sani (official only with Hinkai and without a government post), so the official was able to join the bureaucrats hierarchy system eventually by having the real government post. Hinkai did not have a great deal of meaning here, and only the government post had hierarchy function in the bureaucrats system.
On the other hand, on the Japanese Ritsuryo system, government officials were ranked by Ikai first and given the government post correspond to the Ikai. The hierarchy system by the clans had been existed before the establishment of Ritsuryo system in Japan. Therefore, it was required to restructure each clans as government officials by preserving the former hierarchy when promulgating the government official system at the installation of Ritsuryo. It seemed to be the reasons that the official rank system was introduced with Ikai to be main and government post to be the following, which were different from Tang. This unique Japanese official rank system was called Kani Sotosei.
Specifically, the government post that was possible to assume for each Ikai was strictly established based on the Court Ranking Law. Ikai was absolutely principal in the hierarchy system, so even though an official had achievements and evaluations on an government post, he did not receive promotion on the post like in Tang. The principal was that the official was promoted on Ikai first and then received the qualification to assume the government post equivalent to the Ikai.
However, there were some cases where the equivalent relationship between Ikai and the government post was not established by various reasons. In these cases, it was called 'Gyo' when an official had the government post higher than the equivalent Ikai and 'Shu' with the government post lower than the equivalent Ikai.
In actual operation, there was a tendency to be given higher Ikai in whole. Also, the appointment of Ikai lower than Shorokui (Senior Sixth Rank) was not seen with the obsolescence of Ritsuryo system as the years went by.