Yadayim, Chapter 3, Mishnah 4. This portion of the chapter deals with the rabbinic rule that sacred writings, meaning the scrolls of the books of Tanach, convey ritual impurity to the hands that touch them. Our mishnah asks a narrower question: what about the blank parchment surrounding the text? Does the margin share the status of the writing itself?
A word first about how this decree came about, since the mishnah itself does not spell out the reason. Terumah, the produce set aside for the kohanim, was commonly stored in the same place as a household's scrolls, on the sensible instinct that holy things belong together with holy things. The problem was that a storeroom of grain draws mice, and a mouse chewing its way through such a room draws no distinction between food and parchment. Scrolls were being destroyed. The Sages' solution was indirect but effective: they ruled that sifrei kodesh transmit tumah on the level of sheni letumah, so that anyone who handled them was left with impure hands, and hands in that state render terumah unfit. Once the terumah itself stood to be lost, people stopped keeping the two side by side, and the scrolls were saved. This chapter is accordingly working out the limits of that ruling: which holy writings it covers, and how much of each one.
Now to the wording of our mishnah. "Gilayon shebasefer" refers to the unwritten parchment of a scroll. The mishnah then lists where such empty space is found: "shemilma'alah veshellematah", the margin running above the columns and the margin below them, and "shebatchilah veshebasof", the bare stretch where the scroll opens and the bare stretch where it closes. Of every one of them the mishnah rules "metam'in es hayadayim": they convey impurity to a person's hands. Unwritten parchment, then, is no mere leftover; it belongs to the sefer, and the Sages' decree applies to it just as it applies to the written columns.
Rebbi Yehudah enters a reservation about one item on that list. In his view the margin at the scroll's conclusion is "eino metamei", it transmits no tumah, "ad sheya'aseh lo amud", as long as no roller has yet been fixed to it. While the amud is still missing, that piece of parchment has not been brought into service as part of a completed scroll, and so it has not taken on the status of the sefer.
The Bartenura supplies the scribal measurements that lie behind the mishnah's list. Around each column the sofer leaves parchment unwritten: the width of three fingers along the upper edge, and a tefach, a handbreadth, along the lower one. Empty areas are required as well where the scroll begins and where it finishes. The opening margin must be generous: long enough to encircle the rolled scroll completely, so that what shows on the outside is bare parchment and not the writing itself.
Why does the wrapping margin belong at the start rather than the finish? Because a scroll of this kind is wound in the direction of its beginning, starting from its conclusion. The Bartenura draws a comparison to a mezuzah, whose rolling begins at "echad" and proceeds toward "shema", moving from left to right. That leaves the opposite extremity of the scroll for the amud, and sufficient blank parchment must be reserved there for it.
The Bartenura then adds a significant limitation: what has been described holds for a scroll containing the Chamishah Chumshei Torah alone. Should a person combine Torah, Nevi'im and Kesuvim into a single scroll, the whole arrangement switches around. There the long wrapping margin is left where the scroll finishes, wide enough to go around the entire roll, and the room reserved for the amud is placed at the front.
The reason is a matter of honor. Were such a combined scroll rolled toward its beginning, the Torah would end up as the outermost layer, with Kesuvim innermost and Nevi'im in the middle. That would mean that the Torah, the holiest of the sifrei Tanach, serves as the protective covering for books of lesser sanctity, and the Bartenura calls this a disgrace. Accordingly, the direction in which a scroll is rolled, and the placement of its roller, depend on which books that scroll contains.
The lesson of this mishnah, then, is that sanctity extends beyond the ink. The blank borders of a sacred scroll are part of the scroll, subject to the same decree that was designed to keep our holy writings safe, and even the direction in which the parchment is wound reflects the respect owed to the words it carries.