According to the Mishna (76b) one case where two courtyards cannot join together to make a single eiruv is when there is a wall between them that is ten tefahim (handbreadths) high and four tefahim thick. In such a case, the wall itself is a separate reshut ha-yahid, a private domain (see Massekhet Shabbat), so if there is fruit at the top of the wall, people can go to the top of the wall, eat it – and carry there, as well. The fruit, however, cannot be carried down into the houses in the courtyards.
What if the wall is less than four tefahim thick?
Rav said: In this case, the air of two domains controls it. Since the wall is not broad enough to be regarded a domain of its own, the top of the wall is seen as belonging to both courtyards and is then prohibited to both of them. Accordingly, one may not move anything on top of the wall, even as much as a hair’s breadth.
And Rabbi Yohanan said: These residents of one courtyard may raise food from their courtyard to the top of the wall and eat it there, and they may lower the food from the wall to the courtyard; and those residents of the other courtyard may raise food from their courtyard and eat it there, and they may lower the food from the wall to the courtyard. This is because the wall is considered nonexistent, and its domain is viewed as part of the two courtyards.
Rabbi Yohanan’s reference to an exempt domain (makom petur) is a well-known halakha, and the Gemara reacts with surprise to the suggestion that Rav does not accept it. In explanation, the Gemara distinguishes between rabbinic domains and Biblical domains. With regard to Biblical domains like public and private areas, Rav fully accepts the rule of makom petur. With regard to rabbinic domains, like a courtyard, which Biblically is considered private, and only rabbinically needs to arrange an eiruv, Rav argues that “the Sages reinforced their statements even more than those of the Torah.”
It sounds odd to suggest that the Sages were stricter with rabbinic ordinances than with Biblical prohibitions. Tosafot explain that this does not mean that they are taken more seriously. Rather, recognizing that the public is apt to be more careful about Biblical commands than rabbinic ones, they were more inclined to establish measures to protect the integrity of regulations established by the Rabbis. At the same time the Sages limited this rule and did not apply it in situations that are uncommon or in areas of halakha like monetary matters.