This letter from Priest Saichō (767-822) was sent on November 25, 813 to his disciple Taihan who was staying with Priest Kūkai at Takaosan-ji temple (today's Jingo-ji temple). The letter begins with the sentence "We have not seen each other for a long time", and it is called "Kyūkaku-jō" (long absence letter) according to the first two characters of this opening phrase.
According to the letter, Priest Kūkai had sent a poem to Priest Saichō, but Saichō had not been familiar with one of the books mentioned in the poem. In the letter, Saichō asked his disciple to ask Kūkai about the book in question, as Saichō wanted to compose a poem in response to Kūkai's poem.
Priest Saichō was forty-seven years old when he wrote this letter, but he expresses his courtesy in several ways to Priest Kūkai, who was seven years younger than Saichō. For example, when Saichō mentioned Priest Kūkai, he started a new line in order to pay respect to him.
This letter reveals the friendly relationship between Priest Saichō and Priest Kūkai, and at the same time it shows Priest Saichō's sincere and humble character. The handwriting is clear and decent. This letter is one of the outstanding masterpieces in the history of Japanese calligraphy.
 
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Masterpieces of Nara National Museum. Nara National Museum, 1993, pp.92-93, no.70.