According to the Mishna, there are limitations on the material that can be used for writing a geṭ. For example, all agree that a geṭ cannot be written on something that is connected to the ground, although there is a difference of opinion with regard to a situation where the geṭ was written on something connected to the ground, but was signed only after it was removed from there. Rashi explains this based on an earlier Gemara that requires the geṭ to be something that can be handed from husband to wife with no impediment. Others point to the reason suggested by the Talmud Yerushalmi that explains that a geṭ is called a sefer by the Torah. Just as a book is something unconnected to the ground, so a geṭ cannot be connected to the ground.
In addition to writing surfaces that are attached to the ground, Rabbi Yehudah ben Betera forbids using paper that had been erased or using diftira, since those can be forged easily. The Hakhamim of the Mishna disagree, since – as the Gemara explains – they rely on the witnesses that attest to the validity of the document.
Diftira is a Greek word for an animal hide that has been processed and prepared for writing purposes. It, too, like the erased paper, is a writing surface that lends itself to being easily forged.
In order to understand the rule regarding paper that had been erased, it is important to remember that at that time paper was made from very thin strips of papyrus that were pressed together. Pressing papyrus creates a truly smooth surface, on which erasures are very obvious. Once the top layer has been removed, together with the writing that had been erased, a second or third erasure could be done without it being clear that an additional erasure had taken place.