Kiddushin 49A

גט פשוט, עדיו מתוכו. מקושר, עדיו מאחוריו. פשוט שכתבו עדיו מאחוריו, ומקושר שכתבו עדיו מתוכו ⁠– שניהם פסולים. רבי חנינא בן גמליאל אומר: מקושר שכתבו עדיו מתוכו ⁠– כשר, שיכול לעשותו פשוט. רבן שמעון בן גמליאל אומר: הכל כמנהג המדינה.

In an ordinary document, its witnesses are to sign inside it, i.e., on the written side of the paper. In a folded and tied document, its witnesses are to sign on the back of it. With regard to an ordinary document whose witnesses wrote their signatures on the back of it, or a tied document whose witnesses wrote their signatures inside of it, both of these are not valid. R' Ḥanina ben Gamliel says: A tied document whose witnesses wrote their signatures inside of it is valid, because one can transform it into an ordinary document by untying it. Rabban Shimon ben Gamliel says: Everything is in accordance with regional custom. If an ordinary document is generally used and one wrote a bound one, or vice versa, the document is invalid.

והוינן בה: ותנא קמא לית ליה מנהג המדינה? ואמר רב אשי: באתרא דנהיגי בפשוט ועבד ליה מקושר, אי נמי: באתרא דנהיגי במקושר ועבד ליה פשוט ⁠– כולי עלמא לא פליגי דודאי קפידא.

And we discussed it: And does the first tanna not accept that one should follow the regional custom? It is not reasonable that he should take issue with such a basic concept. And Rav Ashi says that they have a dispute in a case where one instructed a scribe to write a document for him: If they are in a place where the custom is to write an ordinary document, and he made a tied one for him; alternatively, if they are in a place where the custom is to write a tied document, and he made an ordinary one for him; in both of these cases, everyone agrees that he was certainly particular in his instructions to the agent that he should follow the regional custom, and if the latter deviated from the custom the document is invalid.