An open document [has] its witnesses on the inside; and a bound [document has] its witnesses on the outside. An open one whose wit-nesses wrote on the outside, and a bound one whose witnesses wrote on the inside, are both invalid. R' Chanina ben Gamliel says: A bound one whose witnesses wrote on the inside is valid, because he can make it an open one. Rabban Shimon ben Gamliel says: All in accord with the custom of the land.