Japanese vellum; japon
Japanese vellum is a thick handmade paper with a vellum surface. According to John Carter’s ABC for Book-collectors, it is properly hand-made in Japan from the inner bark of the mulberry tree. It has a creamy tone and great strength. Japon is the name given to imitation Japanese vellum, made by treating ordinary paper with various chemicals.
The outside part of the book where the cover meets the spine. (The inside part is the hinge.) As leather ages, it tends to lose its natural oils and become brittle and dry, which often leads to cracking of the joints.