Eruvin, Chapter 4, Mishnah 8. In the previous Mishnah we learned that a person overtaken by nightfall on Shabbos eve while he is still four thousand amos from his home, and who identifies a particular tree or fence, may establish his Shabbos base there, as long as it is a defined and specific spot.
Our Mishnah continues with the same subject:
What should a person do if he cannot establish his Shabbos base at a distant location? The Mishnah lists two such cases:
"Eino makir" - he does not identify any particular point that he could designate as his Shabbos base.
"O she'eino baki bahalachah" - he does not know that he has the option of establishing a Shabbos base two thousand amos away from where he stands.
In such a case, "ve'amar shevisasi bimkomi" - he declares that his Shabbos base will be at the spot where he is actually standing, even though he is in the middle of the road and not at any distinctive place. And thereby, "zachah lo mekomo alpayim amah lechol ruach" - he acquires two thousand amos from the point where he is standing, in every direction.
What is the shape of that boundary?
On this point the Tannaim disagree about the meaning of "in every direction":
"Agulos, divrei Rabbi Chanina ben Antigonus" - the boundary is measured two thousand amos to every side equally, and the resulting shape is a circle.
"Vachachamim omrim meruba, k'tavla meruba'as" - the boundary is measured like a square tablet, "kedei sheyihei niskar lazaviyos" - so that he gains the corners.
The explanation is as follows: according to the Sages he has two thousand amos to the east, to the west, to the north and to the south, but at the corners of the square lies the diagonal, which is longer than two thousand amos. In this way he gains the area of the corners, which he would not have acquired according to the opinion of Rabbi Chanina ben Antigonus, that the boundary is circular.
In summary: someone who does not have a specific place in mind for establishing his Shabbos base, or who is not familiar with this halachah, may say "my Shabbos base is in my place" and acquires two thousand amos in every direction from where he is standing. As to the shape of the boundary they disagree: according to Rabbi Chanina ben Antigonus the boundary is circular, and according to the Sages it is square, "like a square tablet," and in this way a person gains the area of the corners.