Buddhist scriptures written on used letter paper are called "Shōsoku-kyō" (letter sutra). Shōsoku-kyō had often been made between the Heian and Muromachi periods. The purpose of writing on used letter paper was to pray for the dead who had written those letters. 　There were three styles of copying sutras on used letter paper: copying on the reverse side of letters, copying on re-manufactured paper made from the letters, or copying over the letter text. The letter sutra introduced in this article falls into the third category. The copyist cut the original letter, adjusted the rims, beat the paper, decorated it with mica, drew lines, and copied the fourth volume of the Daibirushana-jōbutsu Jimpen-kaji-kyō (Mahāvairocana Bhisaṃbodhi sūtra) in ink between the drawn lines. It is uncommon that this sutra was copied during the Kamakura period as it was the Hoke-kyō (the Lotus sutra) and Jōdo-kyō sutras which were popular for copying on used letter paper during and after the Kamakura period. The letters had been written with kana-characters in light black ink. The lively brush strokes of these kana-characters are elegant and produce an effective contrast against the letters of the sutra which were written with kaisho-type Chinese characters in thick black ink. 
Masterpieces of Nara National Museum. Nara National Museum, 1993, p.82, no.61.