1. There is a dispute about whether Rabbinic prohibitions are suspended when observing them would cause holy items to be desecrated.
2. We do not suspect that one will throw holy writings from one domain to another and thereby transgress carrying on Shabbos.
3. The Gemara instructs scribes how to ensure that the ink drying on their parchment does not get dusty.
4. The Mishnah says that one may carry from a ledge outside his window into his house, and from his house to the ledge.
5. The Gemara explains that the Mishnah refers even to a ledge that is jutting out into Reshus ha'Rabim.
A BIT MORE
1. Rebbi Shimon: Rabbinic prohibitions are always suspended in such cases. Rebbi Yehudah: Rabbinic prohibitions are suspended only in certain situations. (Rabah understands that it is only where many people walking in the public domain will clearly trample the holy writings).
2. This is because it is always forbidden to throw holy writings.
3. They should not turn over the Mezuzah or other holy writing in order to stop dust from landing on the writing, because it is denigrating to the holy writing to do so. Rather they should cover the writing with some sort of covering.
4. This is permitted only when the ledge is ten Tefachim high and four Tefachim wide.
5. However, the Gemara says that one may place vessels on the ledge only if they are the type that will break if they fall down into the public domain. If they are not the type that will break when they fall, a person might go down into the public domain and carry the object back into his house, and thus he may not place them on the ledge in the first place.