At present there remain a few letters which are confirmed being in his handwriting; his letter to Naozumi ARIMA and to Okinaga MATSUI are signed as "Miyamoto Musashi Harunobu," and another letter to Sado no kami (Governor of Sado Province) Nagaoka signed as "Niten."
現在、自筆とみなされている有馬直純宛書状・松井興長宛書状には「宮本武蔵玄信」と記し、長岡佐渡守宛書状には「二天」の号も書いている。