[We] do not write [it] on something attached to the ground. [If] he wrote it on something attached, and [then] detached it, completed it, and gave it to her, it is valid. R' Yehudah invalidates [it], unless its writing and completion are done when [it is] detached.  R' Yehudah ben Beseira says: [We] do not write [it] on paper which has been erased, nor on unfinished parchment, because it can be forged; the Sages, however, validate [these].