Eruvin 56B

תנו רבנן: המרבע את העיר, עושה אותה כמין טבלא מרובעת. וחוזר ומרבע את התחומין, ועושה אותן כמין טבלא מרובעת.

A baraita states: One who squares a city in order to determine its Shabbat limit renders it like a square tablet, and then he also squares the Shabbat boundaries and renders them like a square tablet. Consequently, after squaring the city, he adds additional squares of 2,000 cubits to each of its sides.

וכשהוא מודד, לא ימדוד מאמצע הקרן אלפים אמה מפני שהוא מפסיד את הזויות. אלא מביא טבלא מרובעת שהיא אלפים אמה על אלפים אמה, ומניחה בקרן באלכסונה.

And when he measures the Shabbat limit, he should not measure the 2,000 cubits diagonally from the middle of each corner of the city, because if he were to do so, he would lose the corners, i.e., the limit would extend only 2,000 cubits on the diagonal from each of the corners. Rather, he measures the boundary as though he brought a square tablet that is 2,000 cubits by 2,000 cubits, and places it at each corner at its diagonal.

נמצאת העיר משתכרת ארבע מאות אמות לכאן וארבע מאות אמות לכאן. נמצאו תחומין משתכרין שמונה מאות אמות לכאן ושמונה מאות לכאן. נמצאו העיר ותחומין משתכרין אלף ומאתים לכאן ואלף ומאתים לכאן.

As a result, it will be found that the city gains 400 cubits in this corner and another 400 cubits in the opposite corner. Assuming that the city itself is round and has a diameter of 2,000 cubits, as will be explained below, when the borders of the city are squared, approximately 400 cubits are added to the city at each corner. When one then squares the Shabbat boundaries, it is found that the Shabbat boundaries gain 800 cubits in this corner and 800 cubits in the opposite corner. Consequently, by squaring both the city itself and its Shabbat boundaries, it is found that the city and the Shabbat boundaries together gain 1,200 cubits in this corner and 1,200 cubits in the opposite corner.