These three letters are precious as written materials that represent the relationship between Dengyo Daishi (great teacher of the Buddhism) Saicho, the founder of the Nihon Tendai sect, and Kobo Daishi (a posthumous title of the priest Kukai) Kukai the founder of the Shingon sect; both were the two greatest Buddhist priests in the Heian period.
また、日本天台宗の開祖伝教大師最澄と真言宗の開祖弘法大師空海という平安時代仏教の双璧をなす両雄の交流を示す資料としてもこの3通の存在は貴重である。