Shinkai are ranks granted to subjects that are also given to gods. More precisely, ranks are granted to both humans and gods without distinction, and the ones granted to gods are called shinkai. Thus, its ranking structure is the same for subjects, and are composed of three types - ikai (court rank), kuni (or kunto) (order of merit), and honi (history of Japan).
There are 30 levels in Ikai for humans, ranging from Shoshoige to Shoichii (Senior First Rank), but there are only 15 levels in ikai for gods, ranging from Shorokui (Senior Sixth Rank) to Shoichii. The first record of ikai granted to gods is described in the Nihon-shoki (the oldest chronicles of Japan) as ranks granted to Takechinomiaganimasukamo no kotoshironushinokami, Musanimasu no kami, and Murayanimasumifutsuhime no kami of Yamato Province, who displayed miraculous efficacy during the Jinshin War in July 673 under Emperor Tenmu. In 851, shinkai of Shorokui or higher was granted to the enshrined deities in shrines nationwide.
Kuni comes in 12 ranks for both humans and gods. For humans, it was granted to those displaying deeds under arms, but was granted to others as well since the mid-7th century. The same applies to gods. The first mention of kuni being granted to gods is the Kun hachito granted to Tsukubusuma no kami in Omi Province, who displayed miraculous efficacy during the Revolt of Emi no Oshikatsu in 765. There has been no kuni granted to gods from the 11th century onward.
Honi for humans are granted to the imperial family. There are not many cases of Honi being granted to gods. Exceptions are, in December 749, Ippon was granted to Usa Hachiman no daijin in Buzen Province, and Nihon to its Hime no kami.
Ikai for humans brought privileges such as rights to different offices and provisioning of iden (fields given based upon ikai), but ikai for gods was granted merely as an honor, and more value was placed on shrine ranking than on shinkai.
Granting of shinkai was deliberated in meetings by the court nobles upon application from the Department of Worship and Provinces, and decided after reporting to the Emperor. However, in the Heian period, the Department of Worship and provincial governors frequently granted shinkai to their own. Since medieval times, the Yoshida family also issued shinkai, and these were called sogen senji. Sogen senji were originally issued under imperial sanction by the Emperor, but later, the Yoshida family began issuing them on their own.
Originally, shinkai were granted to shrines. This is because in ancient times, enshrined deities were unique to shrines. When the enshrined deity was moved to another shrine, as a general rule, the shinkai was not inherited, and imperial sanction was necessary to inherit the shinkai. However, with the fall of the ritsuryo system, the shinkai was inherited by the shrine to which the enshrined deity was moved.
For example, the shinkai of the Fushimi Inari Taisha Shrine is 'Shoichii,' and hence, the Inari-jinja Shrines nationwide which were transferred from there, also identify themselves as 'Shoichii.'
The shinkai system was abolished during the Meiji period, but shinkai are seen in the names of some shrines today.