Excerpts from Books and Wikipedia
"The Northern Court (...), also known as the Ashikaga Pretenders or Northern Pretenders, were a set of six pretenders to the throne of Japan during the Nanboku-chō period from 1336 through 1392. The present Imperial House of Japan is descended of the Northern Court emperors. . . . The origins of the Northern Court go back to Emperor Go-Saga, who reigned from 1242 through 1246. Go-Saga was succeeded in turn by two of his sons, Emperor Go-Fukakusa and Emperor Kameyama. On his death bed in 1272, Go-Saga insisted that his sons adopt a plan in which future emperors from the two fraternal lines would ascend the throne in alternating succession. This plan proved to be unworkable, resulting in rival factions and rival claimants to the throne." [Wikipedia]