Eiruvin 4:6

Let’s say that two travelers are limited to four-cubit boundaries but that their boundaries overlap. In such a case, they may meet in the area of overlap and share food there, but neither may take from the other’s area back into his own. If there were three people and the middle person overlapped the two outer people, he can share with them and vice versa but the outer two cannot share with one another. Rabbi Shimon said that this is comparable to three courtyards in a row that open one into the next and each of them also opens to the public domain. If the residents of the two outer courtyards made eiruvs with the residents of the middle courtyard, they can carry to and from the middle courtyard but not from one outer courtyard to the other.

Eiruvin 4:7

If Shabbos arrived while a traveler was on the road and he knew of a nearby landmark like a tree or a fence so he stated, “My Shabbos residence is under it,” he has said nothing (because it’s too vague to be effective). If he said, “My Shabbos residence is at its root,” he can walk 2,000 cubits from his location to the root and 2,000 from the root to his home. In this way he can walk 4,000 cubits after Shabbos arrives.