Mishnah Eruvin 9

¹All the roofs of a town are a single domain, provided no roof is ten handbreadths higher or lower [than the neighboring roof], the words of Rabbi Meir. The Sages say: each one is its own domain. Rabbi Shimon says: roofs, courtyards and karpafs are all one domain with regard to objects that were within them when Shabbat began, but not with regard to objects that were in the house when Shabbat began. ²A large roof close to a small roof: the large one is permitted but that of the small one is forbidden. If [the wall of a] large courtyard which shared a wall with a small courtyard was broken down, the use of the large one is permitted, but that of the smaller one is forbidden, because the gap is like a doorway to the large one. If [the wall of a] courtyard which shared a wall with the public domain was broken down, one who brings from it into a private domain or from a private domain into it is liable, the words of Rabbi Eliezer. The sages say: from it into the public domain or from the public domain into it he is exempt because it is like a karmelit. ³A courtyard [whose walls] were breached from two sides, and so also a house [whose walls] were breached from two of its sides, or an alley from which the cross-beam or side-post was removed:They are permitted for that Shabbat but prohibited for the future, the words of Rabbi Judah. Rabbi Yose says: if they are permitted on that Shabbat they are also permitted for the future and if they are prohibited in the future, they are also prohibited for that Shabbat. ⁴One who builds an upper room on the top of two houses, and so too in the case of viaducts, they may carry underneath them on Shabbat, the words of Rabbi Judah. But the sages prohibit this. Rabbi Judah moreover said: an eruv may be prepared for an alley that is a thoroughfare; But the sages forbid this.