Shinnoke

Seshū Shinnōke (世襲親王家^?) was the collective name for the four cadet branches of the Imperial Household of Japan, which were until 1947 entitled to provide a successor to the Chrysanthemum throne if the main line failed to produce an heir. The heads of these royal houses held the title of imperial prince (親王, shinnō^?), regardless of their genealogical distance from the reigning Emperor, as the term seshū in their designation meant that they were eligible for succession.

The Imperial family of Japan considers itself a single dynasty in unbroken succession; however, the succession has often not been directly from father to son, but has been in the male line within a closely related group of people. In the Muromachi period, Prince Yoshihito, the son of the Northern Emperor Sukō was permitted to establish a parallel lineage to the main imperial line, and took the name Fushimi-no-miya from the location of his palace. Without this permission, the line would be considered commoners, and therefore excluded from the succession. This served politically to cement the reunification of the Northern and Southern Court, but provided insurance in the extreme event that the main imperial line should fail to produce a direct heir and become extinct.

This proved to be a fortunate decision, as in 1428, the son of the 2nd Prince Fushimi-no-miya ascended the throne as Emperor Go-Hanazono.

...
Wikipedia