sentences:  The 18th century Joseon dynasty historian Yu Deukgong advocated the proper study of Balhae as part of Korean history, and coined the term "Northern and Southern States period" to refer to this era.Due to the lack of linguistic evidence, it is difficult to make a definitive conclusion for the linguistic relation between the Balhae and Silla languages.Shoku Nihongi implies that the Balhae language, a Goguryeo language, and Silla language share a close relationship