This is related to the use of pen and ink in ancient times. Writing on rice paper with a brush dipped in ink is easy to faint on the back of the paper. In order to write clean and easy to read, this binding method is adopted. Fishtail marks are printed on the folds of the pages, which should be neatly placed on the left, tidied up, fixed with the cover, and then punched and nailed. To ensure that the pages are fixed. Thread-bound books are different from the books we use now. They don't turn left from right, but from left to right. They are written vertically from right to left.