פירוש על בבא בתרא 10:1

גט פשוט – a plain document similar to ours which are not sewn or folded; and every document is called a Get.

A simple document has the signatures within (at the bottom of the; a sewn document has signatures behind [each fold].
If in a simple document its witnesses signed behind, or if in a sewn document its witnesses signed within, they are invalid. Rabbi Hanina ben Gamaliel says: “If in a sewn document its witnesses signed within, it is valid, since it can be made into a simple document.” Rabban Shimon ben Gamaliel says: “Everything should follow local custom.”
The tenth chapter of Bava Batra deals with laws concerning the proper writing and execution of documents.
An “simple” document is one in which the text is written at the top of the page and the witnesses sign on the bottom, similar to forms used today. A “sewn” document is one in which a few lines of text are written, and then the text is folded over and sewn at the fold and then signed on the back of the document. This process is repeated several times, with each fold being witnessed and sign on the back side of the document. Evidently this was a more difficult type of document to forge. The first opinion in the mishnah states that each type of document must be signed in its customary manner: a simple document on the front and a sewn document on the back.
Rabbi Hanina ben Gamaliel states that a sewn document with signatures on the front is nevertheless valid, and is in essence treated as if it was a simple document.
Rabban Shimon ben Gamaliel states that this law is dependent on the customs of the place. If the custom is to allow sewn documents to be signed on the inside as well as the outside then they are valid; if not they are invalid.

עדיו מתוכו – from the inside, similar to ours that we make.

מקושר מאחוריו – he (i.e., the scribe) writes one line or two and wraps them on the blank part and sews it. And one witnesses inscribes on the wrapping from the outside, and he (i.e., the scribe) goes back and writes two lines from the inside and wraps them on the blank part and the second witnesses inscribes on the wrapping from the outside. And similarly, the third witness. But the Rabbis established a folded document because of stringent Kohanim, because they would write a bill-of-divorcement suddenly to their wives and then regret it and they wouldn’t be able to restore them (i.e., because a Kohen cannot marry a divorced woman – see Leviticus 21:7), and the Rabbis ordained for them a folded bill-of-divorcement which is not easy to write quickly, lest in the midst of it, he is pacified. And just as they ordained/instituted a folded document for a Get, so they instituted for other documents that the Sages did not disagree with.

שניהם פסולין – for they did not act as the Sages ordained.

שיכול לעשותו פשוט – for if the seams would tear and the Get would open, it would be a plain document.

רשב"ג אומר הכל כמנההג המדינה – there is a dispute between Rabban Shimon ben Gamaliel and the Rabbis in a place where there is the practice of [both] a plain document and a folded document. If someone said to him: “Make for me a folded [document]” and he went and he made for him an ordinary document, or “Make for me an ordinary document” and he made for him a folded document, the first Tanna/teacher [of the Mishnah] holds that he is particular, and it is unfit. But Rabban Shimon ben Gamaliel holds that since with both (i.e., the plain document and folded document), it [depends upon] local custom, he is not strict and it is appropriate/fit, but the Halakha is according to the first Tanna/teacher [of the Mishnah] (see Talmud Bava Batra 165a).