"Corners" — Eruvin 51b
רש"י עירובין דף נא/א
פיאות כתיב - דכתיב פאת (נגב) ופאת משמע מרובע
The Gemara tells us that the 2000 amos of Techum Shabbos are derived from the 2000 amos that surround and belong to the cities of the Levi'im. In this context, the Gemara states that Bamidbar 35:5: And you shall measure outside the city, the eastern corner, two thousand amos, teaches us that the "corners" of the 2000 amos square box surrounding a city of the Levi'im are squared, so that the city gains more space at the corners (the diagonal — i.e., 2800 amos). The Gemara then inquires why R' Chanina ben Antigonus, who holds that the 2000 amos of Techum Shabbos are circular does not accept the derivation of corners from the cities of the Levi'im and derive that Techum Shabbos is squared.
In explaining the Gemara's question concerning R' Chanina ben Antigonus's position, Rashi, d.h. Pei'os notes: It is written "Corners" — As it is written (the southern) corner, and corner indicates squared. Although some later source emended Rashi and enclosed the southern in parentheses, clearly some version of Rashi did have the southern. The obvious question (that evidently led to the emendation) is, why does Rashi cite the words: the southern corner, while the Gemara itself cites the words the eastern corner?
Teshuvos Beis Efraim (end of Choshen Mishpat §64) in the name of R' Yitzchok HaLevi of Pressburg addresses this question.
In Pesachim (12b) the Gemara states that at , the sun is at "the corner." Rashi there explains that although in a circle there aren't four corners, but there can be two corners — i.e., if one divides a circle into two arcs, each arc "corner."
Accordingly, Rashi here means to prove that the corners in the context of the city of the Levi'im connote a square. How so? Had the verse just mentioned "corners," we could have understood it to refer to a circle, which can be described as having two corners, each 180 degrees apart from the other — either north and south or east and west. Hence, Rashi stresses that two of the corners mentioned are the one in the Gemara — east and the one he mentions — south. Thus, since the verse mentions corners that are only 90 degrees apart, perforce, it must be referring to a square.